Instrument a new Covered Experience: Create Merge Request

Spawned from gitlab-org/gitlab!196723 (comment 2633337030). Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/548046.

The initial create_merge_request Covered Experience will start tracking from the rails request initiated and follow the background job that finalizes the creation to mark the experience as completed.

1. Start covered experience when request initiated

• Instrument request coming in the Rails controller
• Instrument request coming in the Grape endpoint
• Instrument request coming in the GraphQL mutation

2. Resume covered experience in the background job

• Instrument Sidekiq job NewMergeRequestWorker

3. Enable FF and test the instrumented experience

• gstg
• gprd

The Labkit::CoveredExperience framework will be used for instrumenting.