add plsql function to get a list of pg extensions per logical db

[TLINDEN]

Description

Time and time again we need to check if we are affected by certain vulnerabilities of some postgres extension. To check which extensions are enabled on all logical databases on all postgres instances is tedious manual work.

This PR adds a PLsql function which we use in our default postgres_exporter metrics to have this list easily available all the time.

[eberlep]

@TLINDEN the initDBJob is only run on newly created databases and never updated or run for existing databases. With that in mind: What would happen if we roll out the new query.yaml without the plsql function in existing databases?

[TLINDEN]

@TLINDEN the initDBJob is only run on newly created databases and never updated or run for existing databases. With that in mind: What would happen if we roll out the new query.yaml without the plsql function in existing databases?

You're right. The statement would fail because the function wouldn't exist.

[TLINDEN]

Removed the metric. It's enough to have the function available and query all databases once in a while when needed (e.g. if there's an open CVE for an extension). Keep the plsql function though, because it makes it easier to make this query. It has to be added to existing instances manually, but this is no problem, as there's no metric anymore using it.