Add user_id to v3 sync span; add span for sync token association.

Signed-off-by: Jason Volk <jason@zemos.net>

src/api/client/sync/v3.rs

@@ -115,6 +115,7 @@ type PresenceUpdates = HashMap<OwnedUserId, PresenceEventContent>;
 	level = "debug",
 	skip_all,
 	fields(
+		user_id = %body.sender_user(),
 		since = %body.body.since.as_deref().unwrap_or_default(),
     )
 )]