New github_metrics source

[binarylogic]

I'm opening this to start a broader discussion around plugins and integrations with external services. This idea came about after looking into https://github.com/infinityworks/github-exporter. This is an interesting way to pull data from an API/service on an interval. For this source, specifically, we would pull gauges of star counts, release download counts, repo page view counts, and more.

Open questions:

1. Is this better provided as a plugin? And do we want to explore a plugin system?
2. New cmd or exec source #992 is an interesting way to solve this. You could provide a simple github_stats command that would expose the metrics data when executed.
3. New prometheus source #991 is also an interesting way to solve this with the aforementioned Github exporter.
4. We could explore a generic http source that polls an endpoint and parses the response.

There are a lot of big questions we need to answer before we can begin work on this.

[ghost]

I'm not sure that this still would be called github_metric, but I'd like such source to also generate raw events, such as commits, pull requests, comments, and other event types using the event API.