Cleanly separate the documents metrics.md and metrics-reference.md

[kvosper]

The problem

The purposes of the documents metrics.md and metrics-reference.md have become a little muddled. We are adding metrics in some places and not others and it's not clear which document to read.

Detailed description

metrics.md should be describing how a user can get information from our metrics, whereas metrics-reference.md is supposed to be an exhaustive list giving a description of each metric.

As well as filling in any gaps in metrics-reference.md, we should try to avoid just dumping the names of every metric into metrics.md, instead we should simply say which prefix to look under for particular categories of information and then point readers to metrics-reference.md for more details.

Acceptance criteria

• Add any missing metrics to metrics-reference.md
• Avoid describing exact metrics in metrics.md (exceptions can be made if it is necessary to illustrate an example, but we should avoid overdoing it)

[mikkokar]

Action point: Take a metrics dump from a running instance and update the metrics reference accordingly.

[mikkokar]

Fixed by #469