Add utilization namespaces

[samuel-williams-shopify]

Summary

• add Async::Utilization::Namespace as a registry-like view that composes metric names
• add Registry#namespace for deriving namespaced metric registries
• document namespace usage and add an Unreleased release note

Tests

• PATH="$HOME/.rubies/ruby-3.4.4/bin:$PATH" bundle exec sus
• PATH="$HOME/.rubies/ruby-3.4.4/bin:$PATH" bundle exec rubocop