The log level -3 is the minimum log level that is supported today, smaller than debug(-1) and -2(used to log some HRA related logs).
This commit adds a logging HTTP transport to log HTTP requests and responses to that log level.
It implements http.RoundTripper so that it can log each HTTP request with useful metadata like `from_cache` and `ratelimit_remaining`.
The former is set to `true` only when the logged request's response was served from ARC's in-memory cache.
The latter is set to X-RateLimit-Remaining response header value if and only if the response was served by GitHub, not by ARC's cache.
Yusuke Kuoka