[bitnami/postgresql]: configurable data source credentials for postgresql exporter

[snosratiershad]

Description of the change

Added auth.metricsUsername and auth.metricsPassword which will be used as postgresql exporter data source username and password if user is not empty and not postgres. Also binded to secrets to support password files.

Benefits

I can use my custom user with non-admin permissions and just pg_monitor in postgresql exporter

Possible drawbacks

I've tried to make it backward compatible so previous values will not break. But the metrics user is not autogenerated and may confuse users.

Applicable issues

• fixes [bitnami/postgresql] Configurable DATA_SOURCE_USER and similar from values of postgres exporter #36464

Additional information

Update: currently ready to merge

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)

[aravind4799]

It looks like the get-chart check failed because the chart version hasn't been incremented. Bitnami's automation requires a version bump in Chart.yaml for every change.

[javsalgar]

Hi! Thanks for the PR! Yes, it's mandatory, could you bump the version so we can run some verifications?

[snosratiershad]

@javsalgar @aravind4799. Version bumped. readme gereneted. squashed.