Instrument requests count

[ylecuyer]

Description

For monitoring purpose and in order to create a puma worker killer style plugin (which restart workers after 10k requests) I need to add instrumentation for requests count

I have not yet added test nor the entry in HISTORY.md as I want to be sure this PR wont be rejected before working on this.

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added an entry to History.md if this PR fixes a bug or adds a feature. If it doesn't need an entry to HISTORY.md, I have added [changelog skip] the pull request title.
• I have added appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.

[nateberkopec]

LGTM, I think this will work.

[wjordan]

This PR seems similar to #2061, in fact the code change is nearly identical. That PR is still in-progress because test coverage was requested (#2061 (review)).

[nateberkopec]

I have no problem merging either, just a matter of who figures out the testing issues first 😆

[ylecuyer]

Required tests have been added ;)

[nateberkopec]

Wünderbar!