helm: Support adding extra labels to ServiceMonitors

[lambdanis]

Labels on both tetragon and tetragon-operator ServiceMonitors can be overridden using tetragon[Operator].prometheus.ServiceMonitor.labelsOverride Helm values. However, it's nice to be able to add custom labels without overriding all of the default ones. Let's add such option via extraLabels Helm values.

While here, use only selector labels as selectors in ServiceMonitors.

Related: #1820

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[kkourt]

@lambdanis LGTM, but I will leave merging to you :)