[APM] Add missing settings to documentation (#161603)

### Summary

This PR adds missing APM/Observability settings to the documentation:

- `observability:apmAgentExplorerView`
- `observability:apmAWSLambdaPriceFactor`
- `observability:apmAWSLambdaRequestCostPerMillion`
- `observability:apmEnableContinuousRollups`
- `observability:apmEnableServiceMetrics`
- `observability:apmLabsButton`
- `observability:apmServiceGroupMaxNumberOfServices`
- `observability:apmDefaultServiceEnvironment`

This PR also adds @elastic/obs-docs as a codeowner to
`/x-pack/plugins/observability/server/ui_settings.ts` so that we don't
miss documenting settings moving forward.

Closes #118795.

.github/CODEOWNERS

@@ -874,7 +874,6 @@ x-pack/plugins/infra/server/lib/alerting @elastic/actionable-observability
 /x-pack/test/alerting_api_integration/observability/synthetics_rule.ts @elastic/uptime
 /x-pack/test/alerting_api_integration/observability/index.ts @elastic/uptime
 
-
 # Client Side Monitoring / Uptime (lives in APM directories but owned by Uptime)
 /x-pack/plugins/apm/public/application/uxApp.tsx @elastic/uptime
 /x-pack/plugins/apm/public/components/app/rum_dashboard @elastic/uptime
@@ -884,6 +883,9 @@ x-pack/plugins/infra/server/lib/alerting @elastic/actionable-observability
 /x-pack/plugins/observability_shared/public/components/tour @elastic/platform-onboarding
 /x-pack/test/functional/apps/infra/tour.ts @elastic/platform-onboarding
 
+# Observability settings
+/x-pack/plugins/observability/server/ui_settings.ts @elastic/obs-docs
+
 ### END Observability Plugins
 
 # Presentation

docs/management/advanced-options.asciidoc

@@ -393,6 +393,26 @@ value is 10000.
 [[apm-enable-service-overview]]`apm:enableServiceOverview`::
 When enabled, displays the *Overview* tab for services in *APM*.
 
+[[apm-agent-explorer]]`observability:apmAgentExplorerView`::
+beta:[] Enables the Agent explorer view.
+
+[[apm-aws-price]]`observability:apmAWSLambdaPriceFactor`::
+Set the price per Gb-second for your AWS Lambda functions.
+
+[[apm-aws-request]]`observability:apmAWSLambdaRequestCostPerMillion`::
+Set the AWS Lambda cost per million requests.
+
+[[apm-continuous-rollups]]`observability:apmEnableContinuousRollups`::
+beta:[] When continuous rollups is enabled, the UI will select metrics with the appropriate resolution.
+On larger time ranges, lower resolution metrics will be used, which will improve loading times.
+
+[[apm-enable-service-metrics]]`observability:apmEnableServiceMetrics`::
+beta:[] Enables the usage of service transaction metrics, which are low cardinality metrics that can be used by certain views like the service inventory for faster loading times.
+
+[[observability-apm-labs]]`observability:apmLabsButton`::
+Enable or disable the APM Labs button -- a quick way to enable and disable technical preview features in APM.
+See <<apm-labs>> to learn more.
+
 [[observability-apm-critical-path]]`observability:apmEnableCriticalPath`::
 When enabled, displays the critical path of a trace.
 
@@ -401,9 +421,15 @@ preview:[] When enabled, uses progressive loading of some APM views.
 Data may be requested with a lower sampling rate first, with lower accuracy but faster response times,
 while the unsampled data loads in the background.
 
+[[observability-apm-max-groups]]`observability:apmServiceGroupMaxNumberOfServices`::
+Limit the number of services in a given service group.
+
 [[observability-apm-optimized-sort]]`observability:apmServiceInventoryOptimizedSorting`::
 preview:[] Sorts services without anomaly detection rules on the APM Service inventory page by service name.
 
+[[observability-default-service-env]]`observability:apmDefaultServiceEnvironment`::
+Set the default environment for the APM app. When left empty, data from all environments will be displayed by default.
+
 [[observability-enable-aws-lambda-metrics]]`observability:enableAwsLambdaMetrics`::
 preview:[] Display Amazon Lambda metrics in the service metrics tab.
 

docs/settings/apm-settings.asciidoc

@@ -43,6 +43,7 @@ If you'd like to change any of the default values,
 copy and paste the relevant settings into your `kibana.yml` configuration file.
 Changing these settings may disable features of the APM App.
 
+TIP: More settings are available in the <<observability-advanced-settings,Observability advanced settings>>.
 
 `xpack.apm.maxSuggestions` {ess-icon}::
 Maximum number of suggestions fetched in autocomplete selection boxes. Defaults to `100`.