Poll and report github rate limit API for check-enforcer and other webhooks

Based on my own experience and the frequency of reports in chat, check enforcer delays are something we're starting to see more and more, for both "time to evaluate" and "failure to evaluate." We need to start measuring the reliability of our github automation to have a better idea of whether it's trending worse, how often it approaches failure, and if there is a seasonality to the behavior. Without clear data we have no way to improve the situation or determine whether the current performance is acceptable. Given the difficulty in tracking webhook delivery, polling the github rate limit API seems like a decent first attempt at measurement (and the rate limit API does not affect our rate limits).

[API reference](https://docs.github.com/en/rest/reference/rate-limit)