Feature: implement the volatile counter

[empiredan]

In #931 we've implemented the counter, a type of metric. However, a counter can be volatile, whose value() function will reset the counter atomically after its value is fetched. A volatile counter can also be called as a "recent" counter.

Sometimes "recent" counters are needed, such as the number of recent failed beacons sent from replica server, the count of updating configurations of partitions recently, etc. The "recent" can be considered to be the accumulated count since it has been fetched last.

In most cases, a general (i.e. non-volatile) counter is enough, which means it can also work for "recent" counters. For example, in Prometheus, delta() can be used to compute "recent" count for a general counter. Therefore, declare a counter as volatile only when necessary.