Bad merge behavior for some metrics

We added a feature to merge metrics by commit fe6e5cc0c9c1573de174d1ce7838b449373ae36e ([metrics] Merge metrics by the same attribute), for AtomicGauge type metrics, we sum up of merged metrics, this work for almost all of metrics in Kudu.

But I found a metric that could not be merged like this simply, i.e. "average_diskrowset_height", because it's a "average" value.