Add metrics to specific parts of MergeRequests::AfterCreateService

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Background

We would like to know how each part of the MergeRequests::AfterCreateService performs on their own. This can give us an idea on what we can improve.

Proposal

Add metrics to each distinct responsibility of the MergeRequests::AfterCreateService (like what we did for diffs: #381432 (closed)) using prometheus metrics. Then create a dashboard for it (something like what we did in #388522 (closed)).