Instrument GitHub and GitLab rate limit remaining per code host connection

Created by: ryanslade

Instrument rate limit remaining per code host connection for GitHub. We measure the remaining limit for their core, graphql and seatch APIs separately.

Instrument rate limit remaining per code host connection for GitLab. We only measure the remaining limit for their core API as they only have one limit.

Panels were added to repo-updater with alerts when the rate limit is nearly depleted.

Once we confirm this works as expected we'll remove the tracking and alerting from github-proxy

Closes: https://github.com/sourcegraph/sourcegraph/issues/16455