Provide more structure to docs/metrics.md

[woodsaj]

Metrics2docs searches the code base for metric definitions and builds a docs page using the comments in the code.

In addition to having metrics for the core metrictank binary we also have metrics in other tooling that lives in cmd/*

To make things more clear, we should improve how the metrics are documented, grouping related metrics in separate sections in the current doc, or in individual doc files.

An alternative to the metrics2docs tool would be to embed an API endpoint within our binaries that returns descriptions of all of the metrics that have been registered, similar to the descriptions included when you scrape prometheus metrics.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.