[Helm] Add toggle for MetricsInstance (#7525)

Signed-off-by: Jan-Otto Kröpke <jok@cloudeteer.de>

**What this PR does / why we need it**:

Chart version 3.3.0 introduced a new MetricsInstance CR. The only way to
disable the MetricsInstance is `--set
loki.monitoring.serviceMonitor.metricsInstance=null`, but this raises
helm warning...

```
coalesce.go:220: warning: cannot overwrite table with non table for loki.monitoring.serviceMonitor.metricsInstance (map[annotations:map[] labels:map[] remoteWrite:<nil>])
coalesce.go:220: warning: cannot overwrite table with non table for loki.monitoring.serviceMonitor.metricsInstance (map[annotations:map[] labels:map[] remoteWrite:<nil>])
coalesce.go:220: warning: cannot overwrite table with non table for loki.monitoring.serviceMonitor.metricsInstance (map[annotations:map[] labels:map[] remoteWrite:<nil>])
```

**Checklist**
- [ ] Reviewed the `CONTRIBUTING.md` guide
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

Signed-off-by: Jan-Otto Kröpke <jok@cloudeteer.de>

docs/sources/installation/helm/reference.md

@@ -1968,6 +1968,7 @@ null
 			<td><pre lang="json">
 {
   "annotations": {},
+  "enabled": true,
   "labels": {},
   "remoteWrite": null
 }
@@ -1981,6 +1982,15 @@ null
 			<td><pre lang="json">
 {}
 </pre>
+</td>
+		</tr>
+		<tr>
+			<td>monitoring.serviceMonitor.metricsInstance.enabled</td>
+			<td>bool</td>
+			<td>If enabled, MetricsInstance resources for Grafana Agent Operator are created</td>
+			<td><pre lang="json">
+true
+</pre>
 </td>
 		</tr>
 		<tr>

production/helm/loki/templates/monitoring/metrics-instance.yaml

@@ -1,4 +1,4 @@
-{{- if .Values.monitoring.serviceMonitor.enabled }}
+{{- if and .Values.monitoring.serviceMonitor.enabled .Values.monitoring.serviceMonitor.metricsInstance.enabled  }}
 {{- with .Values.monitoring.serviceMonitor.metricsInstance }}
 apiVersion: monitoring.grafana.com/v1alpha1
 kind: MetricsInstance

production/helm/loki/values.yaml

@@ -633,6 +633,8 @@ monitoring:
     tlsConfig: null
     # -- If defined, will create a MetricsInstance for the Grafana Agent Operator.
     metricsInstance:
+      # -- If enabled, MetricsInstance resources for Grafana Agent Operator are created
+      enabled: true
       # -- MerticsInstance annotations
       annotations: {}
       # -- Additional MatricsInstance labels