Add ObservableGauge metric type

README.md

@@ -22,22 +22,78 @@ Init in your application:
 
 require_once __DIR__ . '/vendor/autoload.php';
 
-// Create a Telemetry instance using OpenTelemetry adapter.
 use Utopia\Telemetry\Adapter\OpenTelemetry;
 
-$telemetry = new OpenTelemetry('http://localhost:4138', 'namespace', 'app', 'unique-instance-name');
-$counter = $telemetry->createUpDownCounter('http.server.active_requests', '{request}');
+$telemetry = new OpenTelemetry('http://localhost:4318/v1/metrics', 'namespace', 'app', 'unique-instance-id');
 
-$counter->add(1);
-$counter->add(2);
-
-// Periodically collect metrics and send them to the configured OpenTelemetry endpoint.
+// Periodically collect and export metrics to the configured OpenTelemetry endpoint.
 $telemetry->collect();
 
 // Example using Swoole
 \Swoole\Timer::tick(60_000, fn () => $telemetry->collect());
 ```
 
+## Metric Types
+
+### Counter
+
+A monotonically increasing counter. Only positive increments are allowed.
+
+```php
+$counter = $telemetry->createCounter('http.server.requests', '{request}', 'Total HTTP requests');
+
+$counter->add(1);
+$counter->add(1, ['method' => 'GET', 'status' => '200']);
+```
+
+### UpDownCounter
+
+A counter that can increase or decrease. Useful for tracking values like active connections.
+
+```php
+$upDownCounter = $telemetry->createUpDownCounter('http.server.active_requests', '{request}', 'Active HTTP requests');
+
+$upDownCounter->add(1);   // request started
+$upDownCounter->add(-1);  // request finished
+```
+
+### Histogram
+
+Records a distribution of values. Useful for measuring latency or payload sizes.
+
+```php
+$histogram = $telemetry->createHistogram('http.server.request.duration', 'ms', 'HTTP request duration');
+
+$histogram->record(142);
+$histogram->record(98.5, ['route' => '/api/users']);
+```
+
+### Gauge
+
+Records an instantaneous measurement. Useful for values that can arbitrarily go up or down.
+
+```php
+$gauge = $telemetry->createGauge('system.memory.usage', 'By', 'Memory usage');
+
+$gauge->record(1_073_741_824);
+$gauge->record(536_870_912, ['host' => 'server-1']);
+```
+
+### ObservableGauge
+
+An asynchronous gauge whose value is collected via a callback at export time. Useful for values that are expensive to compute or come from an external source (e.g. CPU usage, queue depth).
+
+```php
+$observableGauge = $telemetry->createObservableGauge('process.cpu.usage', '%', 'CPU usage');
+
+$observableGauge->observe(function (callable $observer): void {
+    // This callback is invoked each time metrics are collected.
+    $observer(sys_getloadavg()[0] * 100);
+    $observer(72.4, ['core' => '0']);
+    $observer(68.1, ['core' => '1']);
+});
+```
+
 ## System Requirements
 
 Utopia Framework requires PHP 8.0 or later. We recommend using the latest PHP version whenever possible.

src/Telemetry/Adapter.php

@@ -44,5 +44,15 @@ public function createUpDownCounter(
         array $advisory = [],
     ): UpDownCounter;
 
+    /**
+     * @param array<string, mixed> $advisory
+     */
+    public function createObservableGauge(
+        string $name,
+        ?string $unit = null,
+        ?string $description = null,
+        array $advisory = [],
+    ): ObservableGauge;
+
     public function collect(): bool;
 }

src/Telemetry/Adapter/None.php

@@ -6,6 +6,7 @@
 use Utopia\Telemetry\Counter;
 use Utopia\Telemetry\Gauge;
 use Utopia\Telemetry\Histogram;
+use Utopia\Telemetry\ObservableGauge;
 use Utopia\Telemetry\UpDownCounter;
 
 class None implements Adapter
@@ -70,6 +71,18 @@ public function add(float|int $amount, iterable $attributes = []): void
         };
     }
 
+    /**
+     * @param array<string, mixed> $advisory
+     */
+    public function createObservableGauge(string $name, ?string $unit = null, ?string $description = null, array $advisory = []): ObservableGauge
+    {
+        return new class () extends ObservableGauge {
+            public function observe(callable $callback): void
+            {
+            }
+        };
+    }
+
     public function collect(): bool
     {
         return true;

src/Telemetry/Adapter/OpenTelemetry.php

@@ -6,6 +6,7 @@
 use OpenTelemetry\API\Metrics\GaugeInterface;
 use OpenTelemetry\API\Metrics\HistogramInterface;
 use OpenTelemetry\API\Metrics\MeterInterface;
+use OpenTelemetry\API\Metrics\ObserverInterface;
 use OpenTelemetry\API\Metrics\UpDownCounterInterface;
 use OpenTelemetry\Contrib\Otlp\ContentTypes;
 use OpenTelemetry\Contrib\Otlp\MetricExporter;
@@ -25,6 +26,7 @@
 use Utopia\Telemetry\Counter;
 use Utopia\Telemetry\Gauge;
 use Utopia\Telemetry\Histogram;
+use Utopia\Telemetry\ObservableGauge;
 use Utopia\Telemetry\UpDownCounter;
 
 class OpenTelemetry implements Adapter
@@ -34,13 +36,14 @@ class OpenTelemetry implements Adapter
     private MeterInterface $meter;
 
     /**
-     * @var array<class-string, array<string, Counter|UpDownCounter|Histogram|Gauge>>
+     * @var array<class-string, array<string, Counter|UpDownCounter|Histogram|Gauge|ObservableGauge>>
      */
     private array $meterStorage = [
         Counter::class => [],
         UpDownCounter::class => [],
         Histogram::class => [],
         Gauge::class => [],
+        ObservableGauge::class => [],
     ];
 
     /**
@@ -103,12 +106,12 @@ protected function createExporter(TransportInterface $transport): MetricExporter
     }
 
     /**
-     * @template T of Counter|UpDownCounter|Histogram|Gauge
+     * @template T of Counter|UpDownCounter|Histogram|Gauge|ObservableGauge
      * @param class-string<T> $type
      * @param callable(): T $creator
      * @return T
      */
-    private function createMeter(string $type, string $name, callable $creator): Counter|UpDownCounter|Histogram|Gauge
+    private function createMeter(string $type, string $name, callable $creator): Counter|UpDownCounter|Histogram|Gauge|ObservableGauge
     {
         if (! isset($this->meterStorage[$type][$name])) {
             $this->meterStorage[$type][$name] = $creator();
@@ -222,6 +225,39 @@ public function add(float|int $amount, iterable $attributes = []): void
         });
     }
 
+    /**
+     * Create an ObservableGauge metric
+     *
+     * @param array<string, mixed> $advisory
+     */
+    public function createObservableGauge(string $name, ?string $unit = null, ?string $description = null, array $advisory = []): ObservableGauge
+    {
+        return $this->createMeter(ObservableGauge::class, $name, function () use ($name, $unit, $description, $advisory) {
+            $otelGauge = $this->meter->createObservableGauge($name, $unit, $description, $advisory);
+
+            return new class ($otelGauge) extends ObservableGauge {
+                private ?\Closure $callback = null;
+
+                public function __construct(private \OpenTelemetry\API\Metrics\ObservableGaugeInterface $gauge)
+                {
+                    $this->gauge->observe(function (ObserverInterface $observer): void {
+                        if ($this->callback !== null) {
+                            ($this->callback)(function (float|int $value, iterable $attributes = []) use ($observer): void {
+                                /** @var iterable<non-empty-string, array<mixed>|bool|float|int|string|null> $attributes */
+                                $observer->observe($value, $attributes);
+                            });
+                        }
+                    });
+                }
+
+                public function observe(callable $callback): void
+                {
+                    $this->callback = \Closure::fromCallable($callback);
+                }
+            };
+        });
+    }
+
     /**
      * Collect and export metrics
      */

src/Telemetry/Adapter/Test.php

@@ -6,6 +6,7 @@
 use Utopia\Telemetry\Counter;
 use Utopia\Telemetry\Gauge;
 use Utopia\Telemetry\Histogram;
+use Utopia\Telemetry\ObservableGauge;
 use Utopia\Telemetry\UpDownCounter;
 
 /**
@@ -33,6 +34,11 @@ class Test implements Adapter
      */
     public array $upDownCounters = [];
 
+    /**
+     * @var array<string, ObservableGauge>
+     */
+    public array $observableGauges = [];
+
     /**
      * @param array<string, mixed> $advisory
      */
@@ -125,6 +131,23 @@ public function add(float|int $amount, iterable $attributes = []): void
         return $upDownCounter;
     }
 
+    /**
+     * @param array<string, mixed> $advisory
+     */
+    public function createObservableGauge(string $name, ?string $unit = null, ?string $description = null, array $advisory = []): ObservableGauge
+    {
+        $gauge = new class () extends ObservableGauge {
+            public ?\Closure $callback = null;
+
+            public function observe(callable $callback): void
+            {
+                $this->callback = \Closure::fromCallable($callback);
+            }
+        };
+        $this->observableGauges[$name] = $gauge;
+        return $gauge;
+    }
+
     public function collect(): bool
     {
         return true;

src/Telemetry/ObservableGauge.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Utopia\Telemetry;
+
+abstract class ObservableGauge
+{
+    /**
+     * Register an observation callback that will be invoked during collection.
+     *
+     * The callback receives an observer callable that can be called with a value and optional attributes
+     * to record an observation: $observer(float|int $value, iterable $attributes = [])
+     *
+     * @param callable(callable(float|int, iterable<non-empty-string, array<mixed>|bool|float|int|string|null>): void): void $callback
+     */
+    abstract public function observe(callable $callback): void;
+}

tests/Telemetry/Adapter/OpenTelemetryTestCase.php

@@ -8,6 +8,7 @@
 use Utopia\Telemetry\Counter;
 use Utopia\Telemetry\Gauge;
 use Utopia\Telemetry\Histogram;
+use Utopia\Telemetry\ObservableGauge;
 use Utopia\Telemetry\UpDownCounter;
 
 /**
@@ -159,6 +160,38 @@ public function testUpDownCounterAdd(): void
         $this->assertTrue(true);
     }
 
+    public function testCreateObservableGauge(): void
+    {
+        $gauge = $this->adapter->createObservableGauge(
+            name: 'test_observable_gauge',
+            unit: 'bytes',
+            description: 'Test observable gauge metric'
+        );
+
+        $this->assertInstanceOf(ObservableGauge::class, $gauge);
+    }
+
+    public function testObservableGaugeObserve(): void
+    {
+        $gauge = $this->adapter->createObservableGauge('observe_test_gauge');
+
+        // Should not throw
+        $gauge->observe(function (callable $observer): void {
+            $observer(1024);
+            $observer(2048.5, ['host' => 'server-1']);
+        });
+
+        $this->assertTrue(true);
+    }
+
+    public function testObservableGaugeCaching(): void
+    {
+        $gauge1 = $this->adapter->createObservableGauge('cached_observable_gauge');
+        $gauge2 = $this->adapter->createObservableGauge('cached_observable_gauge');
+
+        $this->assertSame($gauge1, $gauge2);
+    }
+
     public function testMeterCaching(): void
     {
         $counter1 = $this->adapter->createCounter('cached_counter');
@@ -229,10 +262,12 @@ public function testNullOptionalParameters(): void
         $histogram = $this->adapter->createHistogram('minimal_histogram');
         $gauge = $this->adapter->createGauge('minimal_gauge');
         $upDownCounter = $this->adapter->createUpDownCounter('minimal_updown');
+        $observableGauge = $this->adapter->createObservableGauge('minimal_observable_gauge');
 
         $this->assertInstanceOf(Counter::class, $counter);
         $this->assertInstanceOf(Histogram::class, $histogram);
         $this->assertInstanceOf(Gauge::class, $gauge);
         $this->assertInstanceOf(UpDownCounter::class, $upDownCounter);
+        $this->assertInstanceOf(ObservableGauge::class, $observableGauge);
     }
 }