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/config/metrics-section_metrics-token.yaml

@@ -0,0 +1,58 @@
+suite: config template | metrics section (metrics token)
+release:
+  name: gitea-unittests
+  namespace: testing
+tests:
+  - it: metrics token is set
+    template: templates/gitea/config.yaml
+    set:
+      gitea:
+        metrics:
+          enabled: true
+          token: "somepassword"
+    asserts:
+      - documentIndex: 0
+        equal:
+          path: stringData.metrics
+          value: |-
+            ENABLED=true
+            TOKEN=somepassword
+  - it: metrics token is empty
+    template: templates/gitea/config.yaml
+    set:
+      gitea:
+        metrics:
+          enabled: true
+          token: ""
+    asserts:
+      - documentIndex: 0
+        equal:
+          path: stringData.metrics
+          value: |-
+            ENABLED=true
+  - it: metrics token is nil
+    template: templates/gitea/config.yaml
+    set:
+      gitea:
+        metrics:
+          enabled: true
+          token:
+    asserts:
+      - documentIndex: 0
+        equal:
+          path: stringData.metrics
+          value: |-
+            ENABLED=true
+  - it: does not configures a token if metrics are disabled
+    template: templates/gitea/config.yaml
+    set:
+      gitea:
+        metrics:
+          enabled: false
+          token: "somepassword"
+    asserts:
+      - documentIndex: 0
+        equal:
+          path: stringData.metrics
+          value: |-
+            ENABLED=false