[APM] Migrate /test/apm_api_integration/tests/event_metadata to be deployment-agnostic API tests

[rmyz]

API	ESS	Serverless	Test files
GET /internal/apm/event_metadata/{processorEvent}/{id}	✅	❌	ESS: ./test/apm_api_integration/tests/event_metadata/event_metadata.spec.ts

[iblancof]

This issue won't be migrated, as we’ve just discovered that it's using the ML index .ml-config, which cannot be modified in serverless (as stated in the parent issue description), causing us problems.

ResponseError: security_exception
   │ 	Root causes:
   │ 		security_exception: action [indices:admin/delete] is unauthorized for user [testing-internal] with effective roles [superuser] on restricted indices [.ml-config], this action is granted by the index privileges [delete_index,manage,all]

cc: @crespocarlos