Add an admin API for server media statistics

[dklimpel]

Add GET /_synapse/admin/v1/statistics/server/media to get statisics about local media usage.
Related to #6094 and #8700

I am not sure if we should mark this API as draft.
Maybe is it better move to GET /_synapse/admin/v1/statistics/server or GET /_synapse/admin/v1/statistics and also add information like number of registered users.
Or should I am add number of registered users to this PR?

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.".
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Signed-off-by: Dirk Klimpel [email protected]

[erikjohnston]

This API doesn't strike me as super useful as is; were you planning on extending it with extra information? I'm mostly thinking that if we are going to end up extending this then it'd be good to know what that would roughly look like up front so that we can make sure the APIs make sense up front 🙂

[clokep]

@dklimpel Were you planning on making further changes to this / rethinking it?

[dklimpel]

Yes, it is in progress. Other tasks have a higher priority at the moment.

[richvdh]

@dklimpel I'm going to close the PR for now. We'd obviously be delighted to reopen it if/when you have time to pick it up again!