Add prometheus metric for raw endpoint rate limiting
Recently we added a RateLimiter for the RawController with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30635/commits .
In addition to logging this issue tracks adding a prometheus metric for how often we are rate limiting. To avoid high cardinality and for privacy reasons we won't be able to use the specific endpoing as a label value, but having a single counter might be nice for general monitoring of this feature.