[postgresql] when specifying a custom user metrics can't be collected anymore

[tbalzer]

Name and Version

bitnami/postgresql 15.5.12

What architecture are you using?

None

What steps will reproduce the bug?

1. Set auth.username to myuser
2. Set metrics.enabled to true
3. Install chart and observe logs of metrics container

Are you using any custom parameters or values?

--set auth.username=myuser --set metrics.enabled=true

What is the expected behavior?

Metrics should be collected from all databases regardless of whether a custom user is specified or not.

What do you see instead?

Once a custom user is specified this user is used to collect metrics and has insufficient permissions.

Additional information

This breaks for us in 15.5.12 as since that version all databases are monitored instead of only the database the custom user has access to.

In my opinion the postgres administrative user should be used to collect metrics. This also solves other related issues like #20247 I believe.

If you agree I'm happy to provide a PR for this.

[carrodher]

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