Add more detailed logging to compute_ctl's shutdown (#3915)

Currently we don't see from the logs, if shutting down tracing takes
long time or not. We do see that shutting down computes gets delayed for
some reason and hits thhe grace period limit. Moving the shutdown
message to slightly later, when we don't have anything else than just
exit left.
## Issue ticket number and link

## Checklist before requesting a review
- [x] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

compute_tools/src/bin/compute_ctl.rs

@@ -203,13 +203,14 @@ fn main() -> Result<()> {
     if delay_exit {
         info!("giving control plane 30s to collect the error before shutdown");
         thread::sleep(Duration::from_secs(30));
-        info!("shutting down");
     }
 
+    info!("shutting down tracing");
     // Shutdown trace pipeline gracefully, so that it has a chance to send any
     // pending traces before we exit.
     tracing_utils::shutdown_tracing();
 
+    info!("shutting down");
     exit(exit_code.unwrap_or(1))
 }