Make healthcheck check a little bit more

[matrixbot]

This issue has been migrated from #11473.

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Currently /health looks like this;

    def render_GET(self, request: Request) -> bytes:
        request.setHeader(b"Content-Type", b"text/plain")
        return b"OK"

Which is functionally equivalent to calling /versions on the endpoint.

I think that this should do a little more than just blindly respond with 'everything is fine', giving me a similar feeling to the following meme;

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Jokes aside, i think that this endpoint should perform or otherwise "check up" on some basic functionality, or otherwise return "not OK" (with 5XX) when some precondition isn't present (which could be defined from other resources).

Maybe this could be linked to an "error counter", which would count the last amount of exceptions in the last minute, and this health resource should then return "not OK" if it passes a threshold.

Other than that, this is open for further ideas.