Add support for reactor metrics with the ProxiedReactor used in worker Complement tests

[MadLittleMods]

Add support for reactor metrics with the ProxiedReactor used in worker Complement tests.

Follow-up to #19383

The ProxiedReactor is a special custom reactor used in the synapse/app/complement_fork_starter.py. It's used by default when using WORKERS=1 ./scripts-dev/complement.sh (see SYNAPSE_USE_EXPERIMENTAL_FORKING_LAUNCHER). The point of the forking launcher is to improve start-up times and the point of the ProxiedReactor is explained in the docstring:

synapse/synapse/app/complement_fork_starter.py

Lines 38 to 56 in 079c52e

	class ProxiedReactor:
	"""
	Twisted tracks the 'installed' reactor as a global variable.
	(Actually, it does some module trickery, but the effect is similar.)
	The default EpollReactor is buggy if it's created before a process is
	forked, then used in the child.
	See https://twistedmatrix.com/trac/ticket/4759#comment:17.
	However, importing certain Twisted modules will automatically create and
	install a reactor if one hasn't already been installed.
	It's not normally possible to re-install a reactor.
	Given the goal of launching workers with fork() to only import the code once,
	this presents a conflict.
	Our work around is to 'install' this ProxiedReactor which prevents Twisted
	from creating and installing one, but which lets us replace the actual reactor
	in use later on.
	"""

(introduced in matrix-org/synapse#13127)

Reproduction instructions

1. Using the Complement scripts with workers: WORKERS=1 COMPLEMENT_DIR=../complement ./scripts-dev/complement.sh ./tests/csapi
2. docker logs complement_csapi_dirty_hs1 2>&1 | grep -i "reactor"
3. With these changes, notice Twisted reactor: ProxiedReactor but no warning about Skipping configuring ReactorLastSeenMetric: unexpected reactor type: <__main__.ProxiedReactor object at 0x7fc0adaaea50>
4. Cleanup:
   • docker stop $(docker ps --all --filter "label=complement_context" --quiet)
   • docker rm $(docker ps --all --filter "label=complement_context" --quiet)

To test that we're actually seeing reactor metrics, I've been using this with the load-test runs in https://github.com/element-hq/synapse-rust-apps/pull/397

Dev notes

Running into this error while building Docker images:

------

 > [internal] load metadata for ghcr.io/astral-sh/uv:python3.13-trixie:
------
Dockerfile:68
--------------------
  66 |     ### Stage 1: builder
  67 |     ###
  68 | >>> FROM ghcr.io/astral-sh/uv:python${PYTHON_VERSION}-${DEBIAN_VERSION} AS builder
  69 |
  70 |     # This silences a warning as uv isn't able to do hardlinks between its cache
--------------------
ERROR: failed to build: failed to solve: failed to fetch oauth token: denied: denied

-> Solution: docker login ghcr.io, see https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry#authenticating-with-a-personal-access-token-classic

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]

Moved ProxiedReactor to it's own file so it actually gets created as synapse.app.complement_fork_proxied_reactor.ProxiedReactor and we can properly do the isinstance(reactor, ProxiedReactor) check in synapse/metrics/_reactor_metrics.py with the same import name.

Previously, it got created as __main__.ProxiedReactor but since we imported it as from synapse.app.complement_fork_starter import ProxiedReactor, the isinstance(reactor, ProxiedReactor) would be False.

[MadLittleMods]

Just moved this logic into a new install_reactor_metrics(...) function which we can re-use.

[MadLittleMods]

The new part of ProxiedReactor is here

[reivilibre]

pretty nice way of doing it

[MadLittleMods]

Thanks for the review @reivilibre 🦎