"memory exceeds quota" log better to be formatted to JSON

[zz-jason]

When the memory threshold is reached, a "memory exceeds quota" log is printed:

 40 // Action logs a warning only once when memory usage exceeds memory quota.
 41 func (a *LogOnExceed) Action(t *Tracker) {
 42     a.mutex.Lock()
 43     defer a.mutex.Unlock()
 44     if !a.acted {
 45         a.acted = true
 46         logutil.Logger(context.Background()).Warn("memory exceeds quota",
 47             zap.Error(errMemExceedThreshold.GenWithStackByArgs(t.label, t.BytesConsumed(), t.bytesLimit, t.String())))
 48     }
 49 }

If we print the tracker tree in JSON format, we can parse and read it easier.