pkg/traceutil: skip subTraceStart/subTraceEnd steps when logging steps (#12262)

SubTraceStart and SubTraceEnd steps are only placeholders, not really
steps, we should skip them when logging the long duration steps,
otherwise these steps will lead to incorrect start time and duration
 of subsequent steps.

pkg/traceutil/trace.go

@@ -192,6 +192,9 @@ func (t *Trace) logInfo(threshold time.Duration) (string, []zap.Field) {
 	}
 	for i := 0; i < len(t.steps); i++ {
 		step := t.steps[i]
+		if step.isSubTraceStart || step.isSubTraceEnd {
+			continue
+		}
 		stepDuration := step.time.Sub(lastStepTime)
 		if stepDuration > threshold {
 			steps = append(steps, fmt.Sprintf("trace[%d] '%v' %s (duration: %v)",