bug 1632621: Rewrite metrics doc for structured logs
bug 1632621: Rewrite metrics doc for structured logs

Rewrite the metrics document and expand it to cover structured logs:

* Describe the difference between statsd metrics and structured logging
* Create an Implementation section, to describe our implementation, the
  issues around high cardinality, and that this is all in progress
* Instead of the narrative flow, split up metrics by web app, task app,
  and datamaps, with some secondary sections as needed, and a new
  section for web app structured logs.
* Split up metrics to a section per metric, with a new style for
  documenting tags, and with cross-references to structure logs
* Reorganize backend and pipeline sections, and add more context. Some
  of this is duplicated on the data flows page, and could be merged.

mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea