Sliding Sync: Add more tracing (element-hq#17514)

Spawning from looking at a couple traces and wanting a little more info.

Follow-up to github.com/element-hq/pull/17501

The changes in this PR allow you to find slow Sliding Sync traces ignoring the
`wait_for_events` time. In Jaeger, you can now filter for the `current_sync_for_user`
operation with `RESULT.result=true` indicating that it actually returned non-empty results.

If you want to find traces for your own user, you can use
`RESULT.result=true ARG.sync_config.user="@MadLittleMods:matrix.org"`

changelog.d/17514.misc

@@ -0,0 +1 @@
+Add more tracing to experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint.