documentation about adding custom metrics seems outdated

[markwort]

Describe the bug
As far as I can tell, the documentation for custom metrics is outdated.
In pgwatch2, the metrics where either in a database, or in /etc/pgwatch/metrics/<metricname>/<min_supported_pg_version>/metric.sql , but it seems to me that there are no longer places in the code that can read these.

Here the old /etc/pgwatch/metrics/ path is still referenced:
https://pgwat.ch/latest/reference/metric_definitions.html#for-yaml-based-setups

However, there seems to be nothing shipped by the pgwatch releases to populate that path.

Expected behavior
There should be a working way to add custom metrics, both for push and pull configuration, and it should be documented.
Maybe the only sane way to do this is to create a custom_metrics.yaml file (similar to metrics.yaml file) and have it loaded by pgwatch.

Best regards
Julian