feat: add metricRelabelings support to ServiceMonitor

[benjohnson-dev]

what

• Adds metricRelabelings value under ServiceMonitor to allow customization of the ingested metrics/labels. See Prometheus Operator for more info - https://github.com/prometheus-operator/prometheus-operator/blob/d2599cfe67beb97b9208e79422457b0f7cde3c4a/Documentation/api.md#endpoint

why

• Prevents potential cardinality issues, allows user control over metric ingestion spend

tests

• diff output of enabled ServiceMonitor with/without new value

--- 219,229 ----
    - port: atlantis
      interval: 30s
      path: /metrics
+     metricRelabelings:
+       - action: drop
+         regex: atlantis_project_apply_execution_.*
+         sourceLabels:
+         - __name__

• ran make unit-test-run-atlantis

### Chart [ atlantis ] ./charts/atlantis

 PASS  test configmap-gitconfig-init for gitconfig	charts/atlantis/tests/configmap-gitconfig-init_test.yaml
 PASS  test pvc	charts/atlantis/tests/pvc_test.yaml
 PASS  test secret-api for api secret	charts/atlantis/tests/secret-api_test.yaml
 PASS  test secret-aws for aws	charts/atlantis/tests/secret-aws_test.yaml
 PASS  test secret-basic-auth for git basic-auth secret	charts/atlantis/tests/secret-basic-auth_test.yaml
 PASS  test secret-gitconfig for gitconfig	charts/atlantis/tests/secret-gitconfig_test.yaml
 PASS  test secret-netrc for netrc	charts/atlantis/tests/secret-netrc_test.yaml
 PASS  test secret-service-account for serviceAccountSecrets	charts/atlantis/tests/secret-service-account_test.yaml
 PASS  test secret-webhook for git webhook secret	charts/atlantis/tests/secret-webhook_test.yaml
 PASS  test service	charts/atlantis/tests/service_test.yaml
 PASS  test statefulset	charts/atlantis/tests/statefulset_test.yaml

Charts:      1 passed, 1 total
Test Suites: 11 passed, 11 total
Tests:       93 passed, 93 total
Snapshot:    0 passed, 0 total
Time:        672.058375ms

references

[GMartinez-Sisti]

Thank you for the contribution @benjohnson-dev!