Improving Circuit Breaker usability

This is a ticket to gather feedback and interest about improving Circuit Breaker usability. If you have opinions or use cases or examples of nicer to use patterns, let us know here.

This ticket does not mean that there's some immediate plan to re-design them, however a discussion with the community about them (or community PRs with design improvements) would be very interesting indeed!

See also:

• https://github.com/akka/akka/issues/18347 should be simpler to use not only with futures
• https://github.com/akka/akka/issues/16617 should expose metrics
• https://github.com/akka/akka/issues/21036 exponential backoff