feat(opentelemetry sink): add native metric to OTLP conversion

[szibis]

Summary

Adds encoding support for converting native Vector metrics to OTLP protobuf format (ExportMetricsServiceRequest), enabling Vector metrics from any source to be sent through OTLP sinks without pre-formatted OTLP structure.

This is the metrics counterpart to log/trace native conversion in #24621.

Related Issues

• Closes Support sending OpenTelemetry metrics #17310 — Support sending OpenTelemetry metrics (primary feature request since 2023, multiple community members waiting)
• Closes Source Host Metrics in OTEL format #24153 — Source Host Metrics in OTEL format (host_metrics → opentelemetry sink now works directly)
• Addresses Support OTLP transform for opentelemetry sink #24515 — Support OTLP transform for opentelemetry sink (eliminates complex VRL for metrics; logs/traces handled by feat(opentelemetry sink): add automatic native log and trace to OTLP conversion #24621)
• Addresses Metrics Opentelemetry sink is failing to push metrics with Bad Request error #23365 — Metrics sink failing with Bad Request (codec: otlp now properly encodes native metrics instead of requiring manual protobuf construction)

User Impact

Before this PR, sending native Vector metrics to an OTLP collector required either:

1. use_otlp_decoding: true on the source (passthrough only, no transforms possible)
2. Complex 50+ line VRL transforms to manually build OTLP protobuf structure (error-prone, caused Metrics Opentelemetry sink is failing to push metrics with Bad Request error #23365)

After this PR:

sources:
  host_metrics:
    type: host_metrics
sinks:
  otel_out:
    type: opentelemetry
    inputs: ["host_metrics"]
    encoding:
      codec: otlp  # Native metrics auto-converted

Conversion Architecture

graph LR
    subgraph "Vector Native Metrics"
        C[Counter]
        G[Gauge]
        H[AggregatedHistogram]
        S[AggregatedSummary]
        D[Distribution]
        Set[Set]
    end
    subgraph "OTLP Protobuf"
        Sum[Sum<br/>monotonic]
        OG[Gauge]
        OH[Histogram]
        OS[Summary]
    end
    C -->|"Incremental→Delta<br/>Absolute→Cumulative"| Sum
    G --> OG
    H -->|"Incremental→Delta<br/>Absolute→Cumulative"| OH
    S --> OS
    D -->|"samples→buckets"| OH
    Set -->|"cardinality count"| OG

Loading

Type Mapping

Vector MetricValue	OTLP Data Type	Temporality Logic
Counter	Sum (monotonic)	Incremental → Delta, Absolute → Cumulative
Gauge	Gauge	—
AggregatedHistogram	Histogram	Incremental → Delta, Absolute → Cumulative
AggregatedSummary	Summary	—
Distribution	Histogram	samples converted to bucket boundaries
Set	Gauge	emits unique value count
Sketch	(dropped with warning)	not directly representable in OTLP

Tag Decomposition

Reverses the decode-path flattening from build_metric_tags:

Tag Pattern	OTLP Destination
resource.*	Resource.attributes[] (prefix stripped)
scope.name	InstrumentationScope.name
scope.version	InstrumentationScope.version
scope.* (other)	InstrumentationScope.attributes[] (prefix stripped)
everything else	DataPoint.attributes[]

Safety

• Zero panics, zero unsafe blocks in encode path
• All numeric casts use From/TryFrom (no bare as casts for narrowing)
• Non-finite (NaN/Infinity) distribution samples are filtered before accumulation
• Sketch metrics produce empty data (no silent corruption with zero gauges)
• internal_log_rate_limit used for rate-limited warnings (Vector convention)

Test Plan

• 47 unit tests covering all metric type conversions
• Roundtrip tests (encode → decode → verify fidelity) for Counter, Gauge, Histogram, Summary
• Tag decomposition: resource, scope, data point attributes, mixed, prefix collision
• Edge cases: zero values, negative values, empty buckets/quantiles, large values, special chars
• NaN/Infinity handling: distribution samples skipped, gauge/counter passthrough, all-NaN distribution
• Sketch metric dropped (no data), zero-rate samples, scope custom attributes
• Non-finite histogram/summary sum passthrough verification
• cargo clippy clean, cargo fmt clean
• CI passes

[buraizu]

Created DOCS-13660 for documentation team review

[szibis]

Companion decode-side fix: #24905 adds decode for missing scope, schema_url, and resource.dropped_attributes_count fields across logs, traces, and metrics. This ensures the fields that this PR's encode path handles are actually populated by the decode side.