[prometheus-postgres-exporter] Use non default database in multi-targets

[thomas-may]

What this PR does / why we need it

This PR introduces the ability to select the database to connect to in a multi-target configuration and have the servicemonitor correctly scrape databases that do not match username.

The multi-target configuration section is completed with the new optional parameter databaseName as follows:

serviceMonitor:
  enabled: true
   # Use multipleTarget mode
  multipleTarget:
    enabled: true
    targets:
      - endpoint: pg01.local
        name: pg01
      - endpoint: pg02.local
        name: pg02
        databaseName: mydb     # <== New optional parameter for selecting database

This configuration produces the following servicemonitor resource:

apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
# ... [redacted]
spec:
  endpoints:
  - metricRelabelings:
    # ... [redacted]
    params:
      auth_module:
      - client.pg01
      target:
      - pg01.local:5432
    path: /probe
    port: http
  - metricRelabelings:
    # ... [redacted]
    params:
      auth_module:
      - client.pg02
      target:
      - pg02.local:5432/mydb     # <== Target references the database in its path
    path: /probe
    port: http
  # ... [redacted]

Special notes for your reviewer

The multi-target feature is currently in beta, but such configuration would be needed for any users relying on a Postgres user dedicated to monitoring purposes, without a database matching its name.

Currently, a workaround is to keep the servicemonitor of this chart disabled and define a custom servicemonitor.

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])