[Synthetics] Port multi-space monitors updates from 9.1

docs/en/observability/synthetics-configuration.asciidoc

@@ -253,6 +253,10 @@ with the {kib} URL for the deployment from which to fetch available locations.
 * Find `Synthetics` in the {kibana-ref}/introduction.html#kibana-navigation-search[global search field] and click *Create monitor*.
 {private-location}s will be listed in _Locations_.
 
+`spaces` (`Array<string>`)::
+The Kibana spaces in which the monitor will be visible. Monitors can be visible in multiple spaces simultaneously. Use `*` to make the monitor visible in all spaces. The project's default space is always included.
+
+
 `throttling` (`boolean` | https://github.com/elastic/synthetics/blob/{synthetics_version}/src/common_types.ts#L194-L198[`ThrottlingOptions`])::
 Control the monitor's download speeds, upload speeds, and latency to simulate your application's behavior on slower or laggier networks. Set to `false` to disable throttling altogether.
 `screenshot` (https://github.com/elastic/synthetics/blob/{synthetics_version}/src/common_types.ts#L192[`ScreenshotOptions`])::

docs/en/observability/synthetics-get-started-ui.asciidoc

@@ -124,7 +124,9 @@ For more information, refer to <<synthetics-recorder>>.
 . Click *Advanced options* to see more ways to configure your monitor.
 +
 ** Use *Data options* to add context to the data coming from your monitors.
+** Use **Maintenance windows** to prevent alerts from being triggered during scheduled downtime.
 ** Use the *Synthetics agent options* to provide fine-tuned configuration for the synthetics agent.
+** Use **Spaces** to select which {kib} spaces the monitor will be visible in. You can select multiple spaces or choose to make the monitor visible in all spaces.
 Read more about available options in <<synthetics-command-reference>>.
 
 . (Optional) Click *Run test* to verify that the test is valid.

docs/en/observability/synthetics-monitor-use.asciidoc

@@ -21,6 +21,7 @@ You'll need to set a few configuration options:
 * **Set the schedule.** Specify the interval at which your tests will run.
 * **Specify where the monitors should run.** You can run monitors on Elastic's global managed testing infrastructure
 or <<synthetics-private-location,create a {private-location}>> to run monitors from your own premises.
+* **Choose which spaces the monitor is visible in.** ({kib} only) You can make the monitor visible in one or more [spaces](/deploy-manage/manage-spaces.md), or use `'*'` for all spaces. Options set in `monitor.use()` override the project-level `spaces` setting in your Synthetics project configuration. For more information, refer to <<synthetics-configuration>>.
 * **Set other options as needed.** There are several other options you can set to customize your implementation including params, tags, screenshot options, throttling options, and more.
 
 Configure each monitor directly in your `journey` code using `monitor.use`.
@@ -54,6 +55,5 @@ journey('Ensure placeholder is correct', ({ page, params }) => {
 });
 ----
 
-For each journey, you can specify its `schedule` and the `locations` in which it runs.
-When those options are not set, Synthetics will use the default values in the global configuration file.
+For each journey, you can specify its `schedule`, the `locations` in which it runs, `spaces` (applies to Stack deployments only), and other options. When an option is not set in `monitor.use()`, Synthetics uses the default from the global configuration file. Options set in `monitor.use()` take precedence over the project-level config—for example, `spaces` here overrides the global `monitor.spaces` setting.
 For more details, refer to <<synthetics-configuration>>.

docs/en/observability/synthetics-reference/lightweight-config/common.asciidoc

@@ -270,6 +270,30 @@ The value defined via the CLI takes precedence over the value defined in the lig
 and the value defined in the lightweight monitor configuration takes precedence over the value defined in _project_ configuration file.
 ====
 
+////////////////////////
+spaces
+////////////////////////
+| [[monitor-spaces]] *`spaces`*
+(<<synthetics-lightweight-data-string,list of strings>>)
+a| The {kib} spaces in which the monitor will be visible. Monitors can be visible in multiple spaces simultaneously. Use `'*'` to make the monitor visible in all spaces. The project's default space is always included. Only applies to Stackdeployments; not applicable to Serverless.
+
+*Examples*:
+
+[source,yaml]
+----
+spaces:
+  - default
+  - team-ops
+----
+
+To make the monitor visible in all spaces:
+
+[source,yaml]
+----
+spaces:
+  - '*'
+----
+
 ////////////////////////
 private_locations
 ////////////////////////