Add 'conn_id' field to `sync_once` span

[erikjohnston]

This is to make it easier to see which sync requests are for which connection when debugging.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.02%. Comparing base (05505a5) to head (ef8e3a7). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4290 +/- ## ======================================= Coverage 85.01% 85.02% ======================================= Files 274 274 Lines 30043 30043 ======================================= + Hits 25542 25543 +1 + Misses 4501 4500 -1 ```

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