Change 'metrics' to 'metrics.enabled' in helm chart documentation

[Kasape]

Description

According to the original documentation of values in README.md, one might think metrics are enabled just by defining the metrics key like this:

metrics:
    serviceMonitor:
        enabled: true
        interval: 30s
    prometheusRule:
        enabled: true

However, templates/service.yaml and other files don't check if the metrics key exists, but how the enabled value is set. The same value is defined in values.yaml. The documentation has been modified to make it clear that the value metrics.enabled must be explicitly defined.

metrics:
    enabled: true
    serviceMonitor:
        enabled: true
        interval: 30s
    prometheusRule:
        enabled: true

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Questions or Comments

[jertel]

Thanks for clarifying this!