Allow labels to be added to serviceMonitor resources

[forestoden]

Is your feature request related to a problem? Please describe.

When using the prometheus operator, one can configure Prometheus to only look at ServiceMonitors that match a particular label. With the thanos-operator, the serviceMonitor fields are only booleans, and I do not see any way to add labels to them to match what prometheus operator would expect.

Describe the solution you'd like to see

A new field serviceMonitorLabels or something to that effect that would add additional labels to the Service Monitor resource.

Describe alternatives you've considered

Additional context