Relax bounds on some metrics_util::registry::Registry methods (#484)

The `Registry` struct definition requires `K: Hashable`, which is
superfluous since it's already required in the impl block.

Moreover, some methods don't require the full `K: Clone + Eq + Hashable`
bounds. Some methods only require `K: Eq + Hashable`, others only
require `K: Hashable`, while others don't require any bounds at all on
`K`.

This commit splits the single impl block into three impl blocks, so
users that have keys that don't satisfy all bounds can still make use of
some methods.