internal: Add logging to observe rate limits of external services

Created by: indradhanush

Recently we've noticed that we see frequent "internal rate limit exceeded" errors on dotcom. Whereas, based on the investigation of the current site-config and the external service config of the cloud default GitHub external service, the rate limit should be infiinite (math.MaxFloat64) as we don't set the requestsPerHour config at all. These logs will help us to get an insight into the current state of the rate limiter for these external services.

COREAPP-218