Add 'cache status/readiness' metric to trafficserver

This is a request to add a 'cache readiness' metric (as a time value) to trafficserver.

Currently, it is difficult to determine the state of the cache volumes unless trafficserver is configured not to serve traffic until the cache volumes have been brought online.

It is desirable to have the ability to poll trafficserver's view of the cache volume(s) state (such as a timestamp of when the cache became ready) before or after the cache volumes become available to monitor state.