Be able to correlate timeouts in reverse-proxy layer in front of Synapse (pull request ID from header) (#13801)

Fix https://github.com/matrix-org/synapse/issues/13685

New config:

```diff
  listeners:
    - port: 8008
      tls: false
      type: http
      x_forwarded: true
+     request_id_header: "cf-ray"
      bind_addresses: ['::1', '127.0.0.1', '0.0.0.0']
```

1 files changed, 10 insertions, 3 deletions

diff --git a/synapse/config/server.py b/synapse/config/server.py
index c91df636d9..f2353ce5fb 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -206,6 +206,7 @@ class HttpListenerConfig:
     resources: List[HttpResourceConfig] = attr.Factory(list)
     additional_resources: Dict[str, dict] = attr.Factory(dict)
     tag: Optional[str] = None
+    request_id_header: Optional[str] = None
 
 
 @attr.s(slots=True, frozen=True, auto_attribs=True)
@@ -520,9 +521,11 @@ class ServerConfig(Config):
         ):
             raise ConfigError("allowed_avatar_mimetypes must be a list")
 
-        self.listeners = [
-            parse_listener_def(i, x) for i, x in enumerate(config.get("listeners", []))
-        ]
+        listeners = config.get("listeners", [])
+        if not isinstance(listeners, list):
+            raise ConfigError("Expected a list", ("listeners",))
+
+        self.listeners = [parse_listener_def(i, x) for i, x in enumerate(listeners)]
 
         # no_tls is not really supported any more, but let's grandfather it in
         # here.
@@ -889,6 +892,9 @@ def read_gc_thresholds(
 
 def parse_listener_def(num: int, listener: Any) -> ListenerConfig:
     """parse a listener config from the config file"""
+    if not isinstance(listener, dict):
+        raise ConfigError("Expected a dictionary", ("listeners", str(num)))
+
     listener_type = listener["type"]
     # Raise a helpful error if direct TCP replication is still configured.
     if listener_type == "replication":
@@ -928,6 +934,7 @@ def parse_listener_def(num: int, listener: Any) -> ListenerConfig:
             resources=resources,
             additional_resources=listener.get("additional_resources", {}),
             tag=listener.get("tag"),
+            request_id_header=listener.get("request_id_header"),
         )
 
     return ListenerConfig(port, bind_addresses, listener_type, tls, http_config)