Imports of package metrics

PathSynopsis
github.com/coreos/etcd/third_party/github.com/rcrowley/go-metrics
ioPackage io provides basic interfaces to I/O primitives.
netPackage net provides a portable interface for network I/O, including TCP/IP, UDP, domain name resolution, and Unix domain sockets.
syncPackage sync provides basic synchronization primitives such as mutual exclusion locks.
timePackage time provides functionality for measuring and displaying time.