Feature: Allow for metric prefix scheme that includes host and/or check name

[Jef Spaleta]

Since we can't easily chain asset based mutators in a set inside sensu handler config, it might be useful to be able to allow this mutator to be configured to prefix metrics names using some sort of template using information extracted from the event itself.

Example(s)

1. when using this mutator with proxy requests to send data into a SumoLogic collector, it maybe useful to prefix the metrics with entity name.

2. Multiple checks checks generate a metrics of the same name. (it can easily happen with something like 'size' or 'time' when checks are running to provide response time profiling information for 3rd party services via something like Nagios check_http. Solution prefix with checkname to disambiguate the information before exporting out as prom metrics format.