Add metrics for monitoring GitHub API rate limit

[int128]

We often face exceeding the rate limit of GitHub API and would like to monitor the status.

This will add the following metrics for monitoring the rate limit:

# HELP github_rate_limit The maximum number of requests you're permitted to make per hour
# TYPE github_rate_limit gauge
github_rate_limit 5000
# HELP github_rate_limit_remaining The number of requests remaining in the current rate limit window
# TYPE github_rate_limit_remaining gauge
github_rate_limit_remaining 4852
# HELP go_gc_duration_seconds A summary of the GC invocation durations.
# TYPE go_gc_duration_seconds summary

See also https://book.kubebuilder.io/reference/metrics.html.

[mumoshu]

Good to know you could do this universally at the Transport-level. Thanks @int128 ☺️

[callum-tait-pbx]

Potentially resolves #196