Support "--auto-discover-databases" feature with postgres_exporter queries

[kwould]

Describe the enhancement you are proposing:
Postgres_exporter has a flag you can enable called --auto-discover-databeses where it allows you to dynamically run queries on the databases that are inside the postgres instance (https://github.com/prometheus-community/postgres_exporter#automatically-discover-databases).
As a workaround to support these queries, they need to be updated to utilize current_database() to add the extra label of which database the query is being ran on. It would be nice if given the current file structure we had db specific and instance specific queries so that it's easier to segment.

I'm happy to work on this, I just wanted to first see if this was something that would be worthwhile.

[keithf4]

I've seen quite a few open issues surrounding this auto-discovery feature so I've been avoiding it myself. Below are just a few of the ones still open

prometheus-community/postgres_exporter#588
prometheus-community/postgres_exporter#753
prometheus-community/postgres_exporter#750
prometheus-community/postgres_exporter#735

[kwould]

Thanks for the quick response @keithf4, anecdotally, most of those issues I saw due to having improperly configured queries. For my use case it would be sufficient enough if queries were segmented by files so that db specific queries lived in a seperate file compared to instance specific ones. This seems to fall with a similar pattern to how global queries are currently constructed.
Would that break anything?

[keithf4]

So we already have our queries segmented to global and per-db. And our per-db queries already have the current_database() specification.

https://github.com/CrunchyData/pgmonitor/blob/development/postgres_exporter/common/queries_per_db.yml

So if you want to try using the --auto-discover-databeses with the exporter that's running the per_db queries, that might just work with how things are and just customizing your sysconfig file.

I don't want to the have the global queries running against all the databases. That just ends up duplicating metrics for every database that exists.

[keithf4]

Did this answer your question on using this feature? I don't think we'll be enabling that option by default in pgMonitor, but I think it should work for the per-db exporter if anyone wants to use it there.

[kwould]

@keithf4 thanks, yea this should be sufficient for us in the meantime