metrics: accept any viable target update

[pared]

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Related to treeverse/dvc#4590

[jorgeorpinel]

We can avoid this new paragraph by changing the beginning of the --targets option desc from

`--targets <paths>` - limit command scope to these metric files. Using -R
to something like
`--targets <paths>` - limit command scope to these metrics files (supports any file, even when not found as `metrics` in `dvc.yaml`). Using `-R`

[jorgeorpinel]

(Notice I also fixed the -R at the end, which should be in back quotes.)

[pared]

And problably we should include that info in dvc metrics show --help

[jorgeorpinel]

Thanks! Left some comments ☝️

If you're unable to further edit this soon, please reopen from a branch directly on this repo instead of a fork so it's easier to take it over.

[efiop]

@pared Let's not forget to update plots docs as well, since the changes are so related.

[pared]

Closing in favor of #1809
Sorry @jorgeorpinel, keep forgetting to create PR's from origin.

[jorgeorpinel]

Yeah no problem @pared wither way it works but if you open from origin it's easier for me to take it over if needed.