Add / remove metric tags transforms

[loony-bean]

Rel #524

Adding static tags will allow ingesting useful meta information, such as env, region, etc.

Removing tags will give users control over tagged metric space, and the resources required to store and process it.

Use cases:

• CloudWatch accepts only 10 tags (aka dimensions); user might want to selectively drop unnecessary tags to make sure important tags are not trimmed
• DataDog is limiting multiple unique tag combinations; dropping tags will provide means to control this resource

[binarylogic]

Yep, we need to solidify our concepts of tags, fields, and how they work across logs and metrics. We brought this up in planning as it is popping up across other work we're doing (#736)

[lukesteensen]

This sounds good to me. For now, I think we go with the simple, independent version that just works for metrics. Maybe down the line we can do some work to reduce duplication with add_fields, but I wouldn't worry about it for now.