Make framework count metrics constant time.

In the master metrics, various framework counters are implemented by a `Gauge` metric that `defers` to a linear scan of the frameworks (eg. MESOS-7747). This means that when we sample the metrics, there will be a N serialized loops over all the frameworks, so the cost of sampling master metrics will be very high for sites that have a large number of frameworks, and sampling the metrics will reduce master responsiveness.