Allow adding our own tags to metrics and events

[ethpran]

Currently: Currently, it looks like the handler reports events using combined_host_tags which is a set of host tags based off the chef run. It doesn't look like the handler supports passing any extra tags beyond these auto-discovered tags.

Desired: It would be nice if the handler allowed us to pass in a set of our own tags that would get added to this list of tags or even override them.