Add listener health (#14747)

Fixes: #8780

changelog.d/14747.feature

@@ -0,0 +1 @@
+Add a dedicated listener configuration for `health` endpoint.

docs/usage/configuration/config_documentation.md

@@ -480,6 +480,12 @@ Valid resource names are:
 
 * `static`: static resources under synapse/static (/_matrix/static). (Mostly useful for 'fallback authentication'.)
 
+* `health`: the [health check endpoint](../../reverse_proxy.md#health-check-endpoint). This endpoint
+  is by default active for all other resources and does not have to be activated separately.
+  This is only useful if you want to use the health endpoint explicitly on a dedicated port or
+  for [workers](../../workers.md) and containers without listener e.g.
+  [application services](../../workers.md#notifying-application-services).
+
 Example configuration #1:
 ```yaml
 listeners:

synapse/app/generic_worker.py

@@ -199,6 +199,9 @@ def _listen_http(self, listener_config: ListenerConfig) -> None:
                             "A 'media' listener is configured but the media"
                             " repository is disabled. Ignoring."
                         )
+                elif name == "health":
+                    # Skip loading, health resource is always included
+                    continue
 
                 if name == "openid" and "federation" not in res.names:
                     # Only load the openid resource separately if federation resource

synapse/app/homeserver.py

@@ -96,6 +96,9 @@ def _listener_http(
                     # Skip loading openid resource if federation is defined
                     # since federation resource will include openid
                     continue
+                if name == "health":
+                    # Skip loading, health resource is always included
+                    continue
                 resources.update(self._configure_named_resource(name, res.compress))
 
         additional_resources = listener_config.http_options.additional_resources