tracing code for copr.buildCopTasks is wrong

[time-and-fate]

Bug Report

Introduced in #40825, the tracing logic for these two parts is apparently wrong.

The StartRegion() is called after the part that needs tracing finished.

This will make the recorded time less than expected and can't cover what we want to trace.

tidb/pkg/store/copr/coprocessor.go

Lines 454 to 469 in b1b0995

	elapsed := time.Since(start)
	if elapsed > time.Millisecond*500 {
	logutil.BgLogger().Warn("buildCopTasks takes too much time",
	zap.Duration("elapsed", elapsed),
	zap.Int("range len", rangesLen),
	zap.Int("task len", len(tasks)))
	}
	if elapsed > time.Millisecond {
	defer tracing.StartRegion(bo.GetCtx(), "copr.buildCopTasks").End()
	}
	if opt.elapsed != nil {
	*opt.elapsed = *opt.elapsed + elapsed
	}
	metrics.TxnRegionsNumHistogramWithCoprocessor.Observe(float64(builder.regionNum()))
	return tasks, nil
	}