The `flush` method sets `start := b.clock.Now()` but later computes
duration with `time.Since(start)` instead of `b.clock.Since(start)` for
the `FlushDuration` metric and the debug log. Line 352 already uses
`b.clock.Since(start)` correctly — this makes the rest consistent.
Test output before fix:
```
flush complete count=100 elapsed=19166h12m30.265728663s reason=scheduled
```
After fix:
```
flush complete count=100 elapsed=0s reason=scheduled
```
Mathias Fredriksson