Remove api_observability monkey-patch plugin

David Dieulivolrequested to merge
ms/remove-api-observability-monkey-patch into master

Context

The api_observability plugin was added as a temporary monkey-patch to measure API call volume. It has served its purpose and we are actively working to move it into the gem natively (gitlab-org/ruby/gems/gitlab-triage!381 (merged)). In the meantime, removing it now is safe — there will be a brief gap in observability coverage.

What's in this MR?

Deletes plugins/api_observability.rb and its spec. No other changes.

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