Document monitoring workers

It doesn't seem to be documented anywhere and means that you suddenly start losing metrics without any obvious reason when you go from monolith to workers (e.g. #7312).

docs/metrics-howto.md

@@ -60,6 +60,31 @@
 
 1.  Restart Prometheus.
 
+## Monitoring workers
+
+To monitor a Synapse installation using
+[workers](https://github.com/matrix-org/synapse/blob/master/docs/workers.md),
+every worker needs to be monitored independently, in addition to
+the main homeserver process. This is because workers don't send
+their metrics to the main homeserver process, and therefore
+expose their own metrics themselves (if they are configured to
+do so).
+
+To allow collecting metrics from a worker, you need to add a
+`metrics` listener to its configuration, by adding the following
+under `worker_listeners`:
+
+```yaml
+ - type: metrics
+   bind_address: ''
+   port: 9101
+```
+
+Feel free to change the `bind_address` and `port` parameters
+as long as the resulting listener can be reached by prometheus.
+With this example, the worker's metrics would then be available
+on `http://127.0.0.1:9101`.
+
 ## Renaming of metrics & deprecation of old names in 1.2
 
 Synapse 1.2 updates the Prometheus metrics to match the naming