Add new TableRenderer for `metrics`.

[daavoo]

metrics are currently handled separately from the existing renderers and using a different input format:

https://github.com/iterative/dvc-render/blob/3196929be5be114c8d4c27a0d446857a657b7a4f/src/dvc_render/html.py#L53-L66

We could implement something like a TableRenderer and use it to render metrics. Making it use datapoints as input like the other renderers.