Deprecate metrics
Deprecate metrics:
- deprecate public functions: `load_metric`, `list_metrics` and `inspect_metric`: docstring and warning
- test deprecation warnings are issues
- deprecate metrics in all docs
- remove mentions to metrics in docs and README
- deprecate internal functions/classes

Maybe we should also stop testing metrics?
_The documentation is not available anymore as the PR was closed or merged._ I mark this as Draft because the deprecated version number needs being updated after the latest release. Perhaps now is the time to also update the `inspect_metric` from `evaluate` with the changes introduced in https://github.com/huggingface/datasets/pull/4433 (cc @lvwerra)  What do you think of including what changes users have to do to switch to `evaluate` in the warning message ?
(basically replace `datasets.load_metric` by `evaluate.load`)

I think it can help users migrate to `evaluate` and silence the warnings