Improve the metrics of store state filter

[rleungx]

We have some metrics to help us locate the reason why the scheduling source or target is filtered. e.g.

There is an item named *-store-state-filter which contains many situations, but we cannot distinguish them through the monitor directly.

The logic of the store state filter can be found in https://github.com/tikv/pd/blob/master/server/schedule/filter/filters.go#L220-L358, The metrics related logic can be found in https://github.com/tikv/pd/blob/master/server/schedule/filter/filters.go#L66-L90