Add `pull_request_dequeued.*` metrics

[vicyap]

This PR adds metrics to capture pull request dequeued events.

When a pull request is dequeued, it could be for a variety of reasons. The reasons are (UNKNOWN_REMOVAL_REASON, MANUAL, MERGE, MERGE_CONFLICT, CI_FAILURE, CI_TIMEOUT, ALREADY_MERGED, QUEUE_CLEARED, ROLL_BACK, BRANCH_PROTECTIONS, GIT_TREE_INVALID, INVALID_MERGE_COMMIT).

I would like to count how often a pull requests is dequeued for reasons like CI_FAILURE.

Adds

• github.actions.pull_request_dequeued.total
• github.actions.pull_request_dequeued.reason.${reason}_total.

Docs: https://docs.github.com/en/webhooks/webhook-events-and-payloads?actionType=dequeued#pull_request

[int128]

Thank you for the contribution!