Allow to set labels on servicemonitor

[wdonne]

What type of PR is this?

feature

Which issue does this PR fix:

#91

What does this PR do / Why do we need it:

It allows the user to set the labels of the generated servicemonitor resource. This is needed because Prometheus uses a label selector, but there is no standard label.

If an issue # is not available please add repro steps and logs showing the issue:

Testing done on this change:

werner@ip-192-168-0-151-21> helm unittest -f 'charts/zot/unittests/*.yaml' charts/zot

### Chart [ zot ] charts/zot

 PASS  configmap checksum in deployment charts/zot/unittests/configmap_checksum_test.yaml
 PASS  test ingress     charts/zot/unittests/ingress_test.yaml
 PASS  secret checksum in deployment    charts/zot/unittests/secret_checksum_test.yaml
 PASS  test servicemonitor      charts/zot/unittests/servicemonitor_test.yaml
 PASS  statefulset tests        charts/zot/unittests/statefulset_test.yaml

Charts:      1 passed, 1 total
Test Suites: 5 passed, 5 total
Tests:       19 passed, 19 total
Snapshot:    1 passed, 1 total
Time:        28.680084ms

Automation added to e2e:

Will this break upgrades or downgrades?

No

Does this PR introduce any user-facing change?:

Yes

The values file now has the property `metrics.serviceMonitor.labels`.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.