Support for remote metrics files

[PeterFogh]

Hi. First of all, thanks for your awesome tool.

I wish that the DVC metric feature supports metric files on remote servers.

An example would be: First we define a remote metric file by the command dvc run -m remote://remote_server/metric.json python model_score.py and then we can print that remote metric file using dvc metrics show.

Currently, DVC just raises an AssertionError when showing remote metrics. I wish that DVC just showed the content of the remote metric file like it would do if the file was stored locally.

Note that, this feature request (like #1654 ) concerns the general request that DVC should the same features for its remote files as it has for its local files.

[efiop]

Closing as stale. Will likely get revisited during #3920 research.