Log merge_action_status when merging MR

What does this MR do and why?

To help debugging a QA test failure (setting MR to MWPS), it's good to know what status is returned when #merge! is called in the controller. This way, we can isolate the cause of the flaky failure.

To do that, we push to Gitlab::ApplicationContext so it can be added to logs for Projects::MergeRequestsController#merge action requests as meta.merge_action_status.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #397495