[bitnami/postgresql] collect metrics as postgres user

[tbalzer]

Description of the change

This change intends to change the credentials used by the metrics container to those of the postgres user, regardless of whether a custom user is also created or not.

Benefits

The postgres user has administrative privileges which allows this user to actually monitor all databases correctly, avoiding issues with permissions like in #29156 and #20247

Possible drawbacks

metrics container has full administrative privileges, but this is also the case if no custom user is created. Using the custom user for this however does not seem to be the right approach. It could be improved further by using a dedicated monitoring/metrics user.

Applicable issues

• fixes [postgresql] when specifying a custom user metrics can't be collected anymore #29156
• fixes [bitnami/postgresql] permission denied for function pg_ls_waldir #20247

Checklist

• Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• Variables are documented in the values.yaml and added to the README.md using readme-generator-for-helm
• Title of the pull request follows this pattern [bitnami/<name_of_the_chart>] Descriptive title
• All commits signed off and in agreement of Developer Certificate of Origin (DCO)

[tbalzer]

Hey @jotamartos , what do you think regarding these changes?

[jotamartos]

Accepting this as this is the default approach when no custom user is defined. Ideally, we should create a separate user for the metrics, but that's something that can be done in a different contribution.