support "conventions" of sensu metric commands

[gdressel]

Most sensu metric commands print the hostname as prefix to any other output, and also support the --scheme parameter to customize this prefix.
It would be nice if the sensu_plugin metric classes would create such an output itself - without any extra (boilerplate) code in implementations/subclasses.

See http://www.programblings.com/2013/11/14/sensu-checks-to-report-metrics/