target-type and unit

[Dieterbe]

@woodsaj re 7f94953
note that target-type and unit are mandatory in metrics2.0
they help in auto-selecting applicable aggregation/consolidation methods (we're not going to keep doing all aggregations for all series when we can deduce that only a few are needed based on the type) as well as setting good defaults for viz.
unit should also be useful down the road to automatically set the right unit in grafana (once grafana gets some metrics2.0 support)
I think now would be a great time to make sure all metrics coming in have the needed metadata set.

[Dieterbe]

@woodsaj see reasoning above. can we make these two tags mandatory again?

[Dieterbe]

fixed in #250