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>

templates/gitea/metrics-secret.yaml

@@ -0,0 +1,12 @@
+{{- if and (.Values.gitea.metrics.enabled) (.Values.gitea.metrics.serviceMonitor.enabled) (.Values.gitea.metrics.token) -}}
+apiVersion: v1
+kind: Secret
+metadata:
+  name: {{ include "gitea.metrics-secret-name" . }}
+  namespace: {{ .Values.namespace | default .Release.Namespace }}
+  labels:
+    {{- include "gitea.labels" . | nindent 4 }}
+type: Opaque
+data:
+  token: {{ .Values.gitea.metrics.token  | b64enc }}
+{{- end }}