Provide a default value for server_name to LoggingContext

[anoadragon453]

This is a backwards-compatibility patch for modules that imported LoggingContext from within Synapse. See matrix-org/synapse-s3-storage-provider#133.

Spawned from a change in #18868, which has not yet gone out in a release. As such, re-uses the changelog from it.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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[MadLittleMods]

While, I didn't forsee this problem when making the change, setting a default encourages people not to set this appropriately even in our own Synapse codebase. We could add a lint for our own codebase 🤔

Should downstream consumers really be using LoggingContext? Probably not

This basically the same reasoning I used in #18989 (comment)

[...] we don't expect (and shouldn't support) these things to be used externally.

I would much rather push harder down this route if we're willing (which means this PR isn't necessary).

[MadLittleMods]

Suggested change

	server_name: str = "SERVER_NAME_NOT_PROVIDED",
	server_name: str = "unknown_server_because_not_provided",

To better match what we have in the codebase:

• unknown_server_from_sentinel_context
• unknown_server_from_no_context
• synapse_module_running_from_unknown_server

---

We could go even further if we add a lint to enforce server_name is set within Synapse as then we know this only happens from third-party code:

Suggested change

	server_name: str = "SERVER_NAME_NOT_PROVIDED",
	server_name: str = "unknown_server_from_third_party_code",

This matches the existing synapse_module_running_from_unknown_server:

Suggested change

	server_name: str = "SERVER_NAME_NOT_PROVIDED",
	server_name: str = "third_party_code_running_from_unknown_server",

We can align on whatever pattern we think is best.

[anoadragon453]

This PR is paused while an alternative solution is explored in matrix-org/synapse-s3-storage-provider#134.