[APM] Make 'sampling_rate' agent central config options available to … (elastic#241908)

Ref: PR elastic/elastic-otel-php#296 implements handling of
`sampling_rate` central config option on the EDOT PHP side

## Summary

This makes one more central-config setting available to EDOT PHP
SDK/agents.
An earlier equivalent PR was: elastic#241048

---------

Co-authored-by: kibanamachine <[email protected]>

Author: 2 people

x-pack/solutions/observability/plugins/apm/common/agent_configuration/setting_definitions/edot_sdk_settings.ts

@@ -118,6 +118,7 @@ export const edotSDKSettings: RawSettingDefinition[] = [
     includeAgents: [
       'opentelemetry/java/elastic',
       'opentelemetry/nodejs/elastic',
+      'opentelemetry/php/elastic',
       'opentelemetry/python/elastic',
     ],
   },

x-pack/solutions/observability/plugins/apm/common/agent_configuration/setting_definitions/index.test.ts

@@ -228,7 +228,7 @@ describe('filterByAgent', () => {
 
     it('opentelemetry/php/elastic', () => {
       expect(getSettingKeysForAgent('opentelemetry/php/elastic')).toEqual(
-        expect.arrayContaining(['logging_level', 'infer_spans'])
+        expect.arrayContaining(['infer_spans', 'logging_level', 'sampling_rate'])
       );
     });