Instrument encode_response with tracing in Sliding Sync requests

[MadLittleMods]

Instrument encode_response with tracing in Sliding Sync requests

Just filling in the hole I see in the trace after current_sync_for_user. In this case, I know that it was probably just serializing all ~133k state events I requested for the Matrix HQ room.

The hole at the end after encode_json_response is already tracked by #17722 and we're adding tracing for it in #18804

Dev notes

POST https://matrix-client.matrix.org/_matrix/client/unstable/org.matrix.simplified_msc3575/sync
{
  "lists": {},
  "room_subscriptions": {
        "!OGEhHVWSdvArJzumhm:matrix.org": {
            "required_state": [ ["*","*"] ],
            "timeline_limit": 50
        }
    }
}

Pull Request Checklist

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• Pull request includes a changelog file. The entry should:
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[MadLittleMods]

This matches the pattern we use elsewhere

synapse/synapse/storage/databases/main/events_worker.py

Lines 664 to 667 in 5c20a60

	set_tag(
	SynapseTags.FUNC_ARG_PREFIX + "event_ids.length",
	str(len(event_ids)),
	)

[MadLittleMods]

This matches what we're doing for legacy /sync

synapse/synapse/rest/client/sync.py

Line 293 in 5c20a60

@trace_with_opname("sync.encode_response")

[MadLittleMods]

Thanks for the review @erikjohnston 🐖