Fix server_name in logging context for multiple Synapse instances in one process

[MadLittleMods]

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 https://github.com/element-hq/synapse-small-hosts/issues/48

Prior art

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

synapse/synapse/config/logger.py

Line 216 in 205d9e4

log_metadata_filter = MetadataFilter({"server_name": config.server.server_name})

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 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.

Todo

• Handle unknown_server_from_sentinel_context
  • Remove sentinel context where we log #18870
  • Store the LoggingContext in a ContextVar #18871

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)