Resolve breaking change to run_as_background_process in module API

[MadLittleMods]

Resolve breaking change to run_as_background_process in module API

Fix #18735

In #18670, we updated run_as_background_process to add a server_name argument. Because this function is directly exported from the Synapse module API, this is a breaking change to any downstream Synapse modules that use run_as_background_process.

This PR shims and deprecates the existing run_as_background_process(...) for modules by providing a stub server_name value and introduces a new ModuleApi.run_as_background_process(...) that covers the server_name logic automagically.

Dev notes

It looks like we already do this for @cached

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)

[anoadragon453]

@MadLittleMods out of interest did you see @reivilibre's suggestion here?

[MadLittleMods]

@MadLittleMods out of interest did you see @reivilibre's suggestion here?

-- @anoadragon453, #18737 (comment)

No. I just solved it how I've been solving it for all of the metrics refactors, passing it in manually.

[anoadragon453]

I see, given the consistency this makes sense.

[anoadragon453]

Docs look excellent. Just a minor suggestion below, and a git conflict to resolve.

Thanks for taking this on, this looks good to me.

And a definite warning to our future selves to not expose internals directly. Another common one is feeding the result of datastore functions directly back in API responses.

[MadLittleMods]

Thanks for the review @anoadragon453 🐄