feat: add type and descriptions to prometheus output
The `/_node/_local/_prometheus` is a missing `TYPE` annotation for
`couchdb_httpd_status_codes`.
In addition, it contains no `HELP` annotations, which
are useful when exploring the metrics, particularly where
metrics do not strictly match those returned by the `_stats` or
`_system` endpoints.
This PR adds the missing `TYPE` annotation and adds `HELP` annotations
to all metrics.
The spec for the prometheus text format is at
https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md,
for reference.
It also adds additional spacing between the metrics series, making it
easier for humans to parse.
## couch_prometheus_util:to_prom/3
`couch_prometheus_util:to_prom/3` is replaced by `couch_prometheus_util:to_prom/4`.
which now expects a description alongside the metric name and type.
## couch_prometheus_util:couch_to_prom/3
`couch_prometheus_util:couch_to_prom/3` now extracts the metrics
description from the metric metadata returned by `couch_stats`.
In some cases, where the metrics are transformed e.g. from multiple
metrics to a single metric with a tag, the description is explicitly
specified to match the new metric semantics.
2 files changed
