Document monitoring workers (matrix-org#7357)

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. matrix-org#7312).

changelog.d/7357.doc

@@ -0,0 +1 @@
+Add documentation on monitoring workers with Prometheus.

docs/metrics-howto.md

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 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, but expose them
+directly (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
+```
+
+The `bind_address` and `port` parameters should be set so that
+the resulting listener can be reached by prometheus, and they
+don't clash with an existing worker.
+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