v0.4.0

• Add Async::Utilization::Namespace for composing registry metric names.
• Async::Utilization::Metric is the primary interface, remove #set, #increment, #decrement and #track from Registry.

v0.3.2

• Better observer state handling.

v0.3.1

• Remove unused thread-local dependency.

v0.3.0

• Introduce Metric#track{...} for increment -> decrement.