feat: support value based label filtering

[acpana]

Overview

Hopefully this is not a "holding it wrong" kinda thing.

Today, go-metrics supports named based label filtering. For instance:

https://github.com/armon/go-metrics/blob/master/metrics_test.go#L465

It'd be useful to support "value" based label filtering. For instance, between two metrics with labels:

notOkLabel := Label{Name: "ok_label", Value: "bad_value"}
okLabel := Label{Name: "ok_label", Value: "good_value"}

not allowing the metric that has a label value equal to bad_value can be useful in cases where I care for some of the label values but not all.