Fixes #374 - Librato error without metric
Fixes #374 - Librato error without metric

Checks if an event being sent to Library has a metric
and errors out if it does not.

eric/riemann,Anvil/riemann,jeanpralo/riemann,eric/riemann,aphyr/riemann,riemann/riemann,bmhatfield/riemann,Anvil/riemann,aphyr/riemann,jamtur01/riemann,jamtur01/riemann,jeanpralo/riemann,abailly/riemann,riemann/riemann,bmhatfield/riemann,abailly/riemann