Need to test this more before merging. This code is really messy at this point. There is probably repeated operations on tags and filters that don't need to be there. Once OpenTSDB is at v2.3 we can remove support for v2.1 and at that time I think it will be easier to redo this code.

There is still an edge case here where if you switch metrics, the two metrics share a tag key, but the value set for that tag is not present in the new metric it will still be there.

However, in that case this isn't hidden and you can just change the value, so I don't care to fix this at this point.