support even badly formed histogram metrics
DRY get_proto2 code

also, make target_type and unit tags more clearly mandatory
no need for target['target_type']

that used to be a side effect of how we created new targets.
the target_type key is only really needed in the tags.
clean up some "what" -> "unit"
support dealing with native proto2 metrics properly

* plugin for native proto2 metrics
* config option to update ES for them or not
* better stats for plugins so we know what's going on