Add some light labels to the Processed request logs

[MadLittleMods]

Add some light labels to the Processed request logs.

It's pretty hard to remember the order of all of these ambiguous numbers. I assume they're not totally labeled already to cut down on the length when scanning with your eyes. This just adds a few hints of what each grouping is.

Spawning from staring at some Synapse logs and cross-referencing the Synapse source code over and over.

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)

[richvdh]

👍, but remember to update the docs to match

[MadLittleMods]

👍, but remember to update the docs to match

-- @richvdh, #19548 (comment)

Updated the docs and added a comment in the code to point to the docs that should be updated whenever that log line changes ✅

[MadLittleMods]

Thanks for the review @sandhose 🐉