feat(service-monitor): support bearer token authentication on metrics endpoint (#719)

### Benefits

Can protect metrics endpoint with `Bearer` token authentication provided by gitea.
see PR #637 for previous discussion.

### Possible drawbacks

No possible drawbacks

### Applicable issues

- fixes #635

### Additional information

```
gitea:
  metrics:
    enabled: true
    token: "somepassword"
    serviceMonitor:
      enabled: true
```

Using above configuration is sufficient to secure /metrics endpoint with bearer token and corresponding ServiceMonitor.

### Checklist

- [x] Parameters are documented in the `values.yaml` and added to the `README.md` using [readme-generator-for-helm](https://github.com/bitnami-labs/readme-generator-for-helm)
- [ ] ~~Breaking changes are documented in the `README.md`~~ Not applicable
- [x] Templating unittests are added

Signed-off-by: Hitesh Nayak <hiteshnayak305@gmail.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/719
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Co-authored-by: Hitesh Nayak <hiteshnayak305@gmail.com>
Co-committed-by: Hitesh Nayak <hiteshnayak305@gmail.com>

unittests/metric-secret/metrics-secret-servicemonitor-disabled.yaml

@@ -0,0 +1,23 @@
+suite: Metrics secret template (monitoring disabled)
+release:
+  name: gitea-unittests
+  namespace: testing
+templates:
+  - templates/gitea/metrics-secret.yaml
+tests:
+  - it: renders nothing if monitoring disabled and gitea.metrics.token empty
+    set:
+      gitea.metrics.enabled: false
+      gitea.metrics.serviceMonitor.enabled: false
+      gitea.metrics.token: ""
+    asserts:
+      - hasDocuments:
+          count: 0
+  - it: renders nothing if monitoring disabled and gitea.metrics.token not empty
+    set:
+      gitea.metrics.enabled: false
+      gitea.metrics.serviceMonitor.enabled: false
+      gitea.metrics.token: "test-token"
+    asserts:
+      - hasDocuments:
+          count: 0

unittests/metric-secret/metrics-secret-servicemonitor-enabled.yaml

@@ -0,0 +1,33 @@
+suite: Metrics secret template (monitoring enabled)
+release:
+  name: gitea-unittests
+  namespace: testing
+templates:
+  - templates/gitea/metrics-secret.yaml
+tests:
+  - it: renders nothing if monitoring enabled and gitea.metrics.token empty
+    set:
+      gitea.metrics.enabled: true
+      gitea.metrics.serviceMonitor.enabled: true
+      gitea.metrics.token: ""
+    asserts:
+      - hasDocuments:
+          count: 0
+  - it: renders Secret if monitoring enabled and gitea.metrics.token not empty
+    set:
+      gitea.metrics.enabled: true
+      gitea.metrics.serviceMonitor.enabled: true
+      gitea.metrics.token: "test-token"
+    asserts:
+      - hasDocuments:
+          count: 1
+      - documentIndex: 0
+        containsDocument:
+          kind: Secret
+          apiVersion: v1
+          name: gitea-unittests-metrics-secret
+      - isNotNullOrEmpty:
+          path: metadata.labels
+      - equal:
+          path: data.token
+          value: "dGVzdC10b2tlbg=="