plots/metrics: accept any viable target

[pared]

Running:

#!/bin/bash

rm -rf repo
mkdir repo

pushd repo

git init --quiet
dvc init --quiet


echo "{'metric':0.95}" >> metric.json
git add -A
git commit -am "initial"
git tag v1


echo "{'metric':0.96}" >> metric.json
git add -A
git commit -am "second"
git tag v2

dvc metrics diff v1 v2 -v --targets metric.json

yields no result. metrics diff/show should be able to handle non-metrics targets that can be interpreted by our parsers.

[efiop]

For the record: same for plots

[dmpetrov]

dvc params diff should follow the same logic. But first, --targets needs to be introduced for params.

[dmpetrov]

Metrics/plots/params difference for Git-files is a useful scenario even outside of the data-files or pipeline context. It feels like it is a separate functionality in DVC that needs to be fully supported.

For example, it might be used in CI/CD to compare metrics/plots/params with master or previous commits in a repo when DVC is not used to generate reports like this - iterative/cml_dvc_case#4

Additional scenarios to support:

1. metrics/plots/params diff for not-DVC repos (with --targets specified).
2. metrics/plots/params diff for DVC repos without any pipelines (without --targets specified). So, a dummy\phony stage might be needed.