metrics: make metrics endpoint toggleable

docs/examples/kustomize/sqlite/statefulset.yaml

@@ -31,6 +31,11 @@ spec:
                 configMapKeyRef:
                   name: headscale-config
                   key: metrics_listen_addr
+            - name: METRICS_ENABLED
+              valueFrom:
+                configMapKeyRef:
+                  name: headscale-config
+                  key: metrics_enabled
             - name: DERP_MAP_PATH
               value: /vol/config/derp.yaml
             - name: EPHEMERAL_NODE_INACTIVITY_TIMEOUT