add metrics documentation for new, more granular mount & unmount time metrics
add metrics documentation for new, more granular mount & unmount time metrics

Signed-off-by: Julian Hjortshoj <a84af1ff40fcfa3bc4844358f827e17a8b38c426@emc.com>

cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release