Feature(new_metrics): make the counter increment monotonically

[empiredan]

In #931 we've implemented the counter that can both increase and decrease. However, in Prometheus a counter can only increase monotonically. If the counter decreases, functions of Prometheus such as rate() and increase() cannot be used to compute QPS and "recent" count.

In consideration of the problem, this kind of counter is moved to the gauge. Therefore, the gauge is added with increment() and decrement() functions, which can only be used for integral types.