## This PR
1. Move stats generation after trace obfuscation, which is the correct
order as suggested by Trace Agent team. Right now stats generation is
before trace obfuscation.
2. Also generate trace stats for OTLP agent. Right now we only do it for
trace agent.

## Architecture
Copied from #842
<img width="1296" height="674" alt="image"
src="https://github.com/user-attachments/assets/2d4cb925-6cfc-4581-8ed6-6bd87cf0d87a"
/>

## Testing
Tested in the next PR
#856, which
implements stats concentrator. Trace stats appeared in Datadog.
<img width="538" height="317" alt="image"
src="https://github.com/user-attachments/assets/48b849cc-2413-41d5-8576-5ff657c21a0f"
/>


## Next steps
1. Implement `StatsConcentrator`
2. Rename for clarity:
  - `SendingTraceStatsProcessor` -> `TraceStatsGenerator`
  - `stats_sender` -> `stats_generator`
3. Small refactor: consider passing around `stats_sender` instead of
`stats_concentrator_handle`. Right now
`SendingTraceStatsProcessor::new()` is called in three places. It might
be possible to call it only once then pass it around.

## Notes
Jira: https://datadoghq.atlassian.net/browse/SVLS-7593