Add gitaly_calls to API structured logs

What does this MR do?

Mirrors the gitaly_call metric from the web structured access logs on the API structured logs.

Adding this allows us to analyse n+1 issues on the grape API in the same way that we can on rails routes.

This will allow us to discover and resolve issues such as https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18960#note_98714897

What are the relevant issue numbers?

Tangentially related to https://gitlab.com/gitlab-org/gitlab-ce/issues/50993

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides