Metrics integration

[erikh]

The volmaster, volsupervisor, and volplugin should all expose metrics endpoints over their transports, e.g. volplugin serves over the unix socket and volmaster can serve over http. volsupervisor should probably publish over a unix socket as well to avoid having to expose a network endpoint.

Metrics to expose:

• Total # of Mounts over run time
• Total # of Active Mounts
• Total space used
• Per-volume space information
• Per-volume IO information
• all the statistics from our info package