Re-enable HLL event `i_search_paid` metric

What does this MR do?

Functionality is behind the :search_track_unique_users feature flag

This MR re-enables i_search_paid usage ping metric, which was introduced in !40134 (merged) and disabled via !40842 (merged)

Originally we were calling feature_available?(:elastic_search) on a project or group, but it resulted in ArgumentError: Use License.feature_available? for features that cannot be restricted to only a subset of projects or namespaces on production even though it worked on a local machine.

To resolve that we will separate conditions for Gitlab.com and for self-managed customers:

• Gitlab.com: search_service.use_elasticsearch?
• Self-managed: License.feature_available?(:elastic_search)

#238116 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team