Add optional ServiceMonitor to Helm chart

[hafe]

Is your enhancement request related to a problem? Please describe.
Helm chart users are required to add extra resources to get monitoring to work

What would you like to be added:
An optional ServiceMonitor resource in the Helm chart

Why this is needed:
To simplify for users

Additional context
See for example the cert-manager Helm chart that provides a ServiceMonitor:
https://github.com/cert-manager/cert-manager/blob/master/deploy/charts/cert-manager/templates/servicemonitor.yaml

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