Be mindful of other logging context filters in 3rd-party code

[MadLittleMods]

Be mindful that Synapse can be run alongside other code in the same Python process. We shouldn't overwrite fields on given log record unless we know it's relevant to Synapse.

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

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)

[MadLittleMods]

Is the abstraction worth it? Should I just inline the usage?

Originally, I thought I would have to use it more for all of the request attributes below but turns out we can do a little optimization to avoid it.