etcdserver: add more detailed traces on linearized reading (#12335)

To improve debuggability of `agreement among raft nodes before
linearized reading`, we added some tracing inside
`linearizableReadLoop`.

This will allow us to know the timing of `s.r.ReadIndex` vs
`s.applyWait.Wait(rs.Index)`.

pkg/traceutil/trace.go

@@ -157,6 +157,13 @@ func (t *Trace) LogIfLong(threshold time.Duration) {
 	}
 }
 
+// LogAllStepsIfLong dumps all logs if the duration is longer than threshold
+func (t *Trace) LogAllStepsIfLong(threshold time.Duration) {
+	if time.Since(t.startTime) > threshold {
+		t.LogWithStepThreshold(0)
+	}
+}
+
 // LogWithStepThreshold only dumps step whose duration is longer than step threshold
 func (t *Trace) LogWithStepThreshold(threshold time.Duration) {
 	msg, fs := t.logInfo(threshold)