Use instrumentation classes for Service Ping generation

Now we can use only instrumentation classes to add a new metric to
Service Ping, without changing usage_data.rb.

Changelog: changed

config/feature_flags/development/merge_service_ping_instrumented_metrics.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/352964
 milestone: '14.8'
 type: development
 group: group::product intelligence
-default_enabled: false
+default_enabled: true