Fix server_name in logging context for multiple Synapse instances in one process (#18868)

### Background

As part of Element's plan to support a light form of vhosting (virtual
host) (multiple instances of Synapse in the same Python process), we're
currently diving into the details and implications of running multiple
instances of Synapse in the same Python process.

"Per-tenant logging" tracked internally by
element-hq/synapse-small-hosts#48

### Prior art

Previously, we exposed `server_name` by providing a static logging
`MetadataFilter` that injected the values:


https://github.com/element-hq/synapse/blob/205d9e4fc4774850f34971469ae500e70119d17a/synapse/config/logger.py#L216

While this can work fine for the normal case of one Synapse instance per
Python process, this configures things globally and isn't compatible
when we try to start multiple Synapse instances because each subsequent
tenant will overwrite the previous tenant.


### What does this PR do?

We remove the `MetadataFilter` and replace it by tracking the
`server_name` in the `LoggingContext` and expose it with our existing
[`LoggingContextFilter`](https://github.com/element-hq/synapse/blob/205d9e4fc4774850f34971469ae500e70119d17a/synapse/logging/context.py#L584-L622)
that we already use to expose information about the `request`.

This means that the `server_name` value follows wherever we log as
expected even when we have multiple Synapse instances running in the
same process.


### A note on logcontext

Anywhere, Synapse mistakenly uses the `sentinel` logcontext to log
something, we won't know which server sent the log. We've been fixing up
`sentinel` logcontext usage as tracked by
#18905

Any further `sentinel` logcontext usage we find in the future can be
fixed piecemeal as normal.


https://github.com/element-hq/synapse/blob/d2a966f922fdc95bc86f7fe55b7b54a9ab3f25c1/docs/log_contexts.md#L71-L81


### Testing strategy

1. Adjust your logging config to include `%(server_name)s` in the format
    ```yaml
    formatters:
        precise:
format: '%(asctime)s - %(server_name)s - %(name)s - %(lineno)d -
%(levelname)s - %(request)s - %(message)s'
    ```
1. Start Synapse: `poetry run synapse_homeserver --config-path
homeserver.yaml`
1. Make some requests (`curl
http://localhost:8008/_matrix/client/versions`, etc)
1. Open the homeserver logs and notice the `server_name` in the logs as
expected. `unknown_server_from_sentinel_context` is expected for the
`sentinel` logcontext (things outside of Synapse).

Author: MadLittleMods

changelog.d/18868.misc

@@ -0,0 +1 @@
+Fix `server_name` in logging context for multiple Synapse instances in one process.

synapse/app/_base.py

@@ -599,7 +599,7 @@ def run_sighup(*args: Any, **kwargs: Any) -> None:
     hs.get_pusherpool().start()
 
     def log_shutdown() -> None:
-        with LoggingContext("log_shutdown"):
+        with LoggingContext(name="log_shutdown", server_name=server_name):
             logger.info("Shutting down...")
 
     # Log when we start the shut down process.

synapse/app/admin_cmd.py

@@ -329,7 +329,7 @@ def start(config: HomeServerConfig, args: argparse.Namespace) -> None:
     # command.
 
     async def run() -> None:
-        with LoggingContext(name="command"):
+        with LoggingContext(name="command", server_name=config.server.server_name):
             await _base.start(ss)
             await args.func(ss, args)
 
@@ -342,5 +342,5 @@ async def run() -> None:
 
 if __name__ == "__main__":
     homeserver_config, args = load_config(sys.argv[1:])
-    with LoggingContext(name="main"):
+    with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
         start(homeserver_config, args)

synapse/app/appservice.py

@@ -27,7 +27,7 @@
 
 def main() -> None:
     homeserver_config = load_config(sys.argv[1:])
-    with LoggingContext(name="main"):
+    with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
         start(homeserver_config)
 
 

synapse/app/client_reader.py

@@ -27,7 +27,7 @@
 
 def main() -> None:
     homeserver_config = load_config(sys.argv[1:])
-    with LoggingContext(name="main"):
+    with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
         start(homeserver_config)
 
 

synapse/app/event_creator.py

@@ -26,7 +26,7 @@
 
 def main() -> None:
     homeserver_config = load_config(sys.argv[1:])
-    with LoggingContext(name="main"):
+    with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
         start(homeserver_config)
 
 

synapse/app/federation_reader.py

@@ -27,7 +27,7 @@
 
 def main() -> None:
     homeserver_config = load_config(sys.argv[1:])
-    with LoggingContext(name="main"):
+    with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
         start(homeserver_config)
 
 

synapse/app/federation_sender.py

@@ -27,7 +27,7 @@
 
 def main() -> None:
     homeserver_config = load_config(sys.argv[1:])
-    with LoggingContext(name="main"):
+    with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
         start(homeserver_config)
 
 

synapse/app/frontend_proxy.py

@@ -27,7 +27,7 @@
 
 def main() -> None:
     homeserver_config = load_config(sys.argv[1:])
-    with LoggingContext(name="main"):
+    with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
         start(homeserver_config)
 
 

synapse/app/generic_worker.py

@@ -386,7 +386,7 @@ async def start() -> None:
 
 def main() -> None:
     homeserver_config = load_config(sys.argv[1:])
-    with LoggingContext(name="main"):
+    with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
         start(homeserver_config)