Add TOML support for metrics

[flippedcoder]

Right now, there is only TOML file support for params files. We need to add TOML support for metrics as well.

Here's a link to the Discord question that brought this up.

[pmrowla]

Is this a high priority issue? We don't get many requests for TOML metrics support (that one discord question is the only one in the last couple of years as far as I can tell)

[dberenbaum]

It's not high priority, but a user did request it, so @flippedcoder volunteered to open an issue for it.

[skshetry]

Anyone looking for contributing, the place to add the support is here:
https://github.com/iterative/dvc/blob/6c646a70703cc68991eeb3232748b39c759c942c/dvc/repo/metrics/show.py#L70

Please compare that to the params which already supports toml file:
https://github.com/iterative/dvc/blob/6c646a70703cc68991eeb3232748b39c759c942c/dvc/repo/params/show.py#L53-L55

Also note that, LOADERS also supports reading .py files which we might need to forbid here, if we use it in the metrics.