metrics diff: accept multiple revisions (like plots diff)

[jorgeorpinel]

Context: https://discuss.dvc.org/t/fill-back-metrics/441

Related: #4210

[dmpetrov]

It should be a separate command like dvc experiment show HEAD HEAD^ ab3132d
It might be a part of #2799

[jorgeorpinel]

Will metrics and plots be combined into that single command @dmpetrov ?

[jorgeorpinel]

I guess it can be considered a duplicate of #4210 — I opened this from the same context.

[efiop]

Will metrics and plots be combined into that single command @dmpetrov ?

@jorgeorpinel Yep, it is alredy in a hidden dvc experiments show/diff.

[jorgeorpinel]

Nice. But that will replace metrics and plots? (just to be extra clear)

[efiop]

@jorgeorpinel It won't, it will be an addition.