Rewrite and reorganize metrics

[MattDodsonEnglish]

The metrics page has a lot of info. Some things we could do to make the flow better:

• Split the reference off to its own page (like with options)
  • Add grpc and websockets metrics (Update built-in metrics  #471)
  • Possibly make a gatsby component to place these references in two places: in their respective API doc, and together on a single page (Investigate ways to template and reuse content #797)
  • Link reference to or from results output, since metrics are just results ouput (or move metrics to that section)
• Make new how to page for custom metrics:
  • Move examples for custom metric types there
• Do a bit more to help readers understand these metrics
  • What info does a gauge convey that a trend cannot?