--show-json support for dvc metrics show

[DavidGOrtega]

The current output is difficult to be handled. Specially with multiple metrics json files.
Since some metrics ops offers the --show-json I would be great if that were supported.

[efiop]

@DavidGOrtega @JIoJIaJIu Guys, talked with @dmpetrov and he advised not to implement this until we redesign dvc metrics show as a whole. So removing from this sprint for now.

[dmpetrov]

It looks like the current dvc metrics diff if enough. This info can be extracted from there.

Closing.