Metric to monitor Github API usage

[pgmillon]

Hi,
In order to monitor Github API usage and prevent API rate limiting it would be great to have a simple metrics exposed.
This could be really helpful on highly used Jenkins instances that have a combination of multibranch pipelines & shared library on Github.
See: https://issues.jenkins-ci.org/browse/JENKINS-36121
Maybe something inside Requester.buildRequest ?

[bitwiseman]

The metrics are exposed:
https://github.com/github-api/github-api/blob/master/src/main/java/org/kohsuke/github/GitHub.java#L317-L373

Perhaps they could be documented better. Or are you looking for this library to implement the rate limit checking like this: https://github.com/jenkinsci/github-branch-source-plugin/blob/1c2c688b50f70da1d237065c31ec69c01d062fb9/src/main/java/org/jenkinsci/plugins/github_branch_source/Connector.java#L572

[bitwiseman]

The approach should be simpler to start with. And then additionally we should implement a RateLimitStrategy class that will automatically check and attempt to throttle requests to avoid going over. It would save clients from having to pepper their code with checkRateLimit() everywhere.

[bitwiseman]

#601

[bitwiseman]

Also added a RateLimitChecker extension point to the API.