Rethink monitoring of remaining code host rate limits

Created by: tsenart

With the introduction of user added external services, we now have a potentially huge number of tokens to monitor. We can't add each token to the src_github_rate_limit_remaining metric, because that's too high cardinality.

As a result of the current state of things, the alert [Cloud] [CRITICAL] github-proxy: less than 500 remaining calls to GitHub before hitting the rate limit for 5m0s is incorrect, because the metric flips up and down, through multiple tokens instead of one.

We could just capture a percentage of external services whose tokens quotas are below a certain threshold. Or monitor only site level external services.