Make traces and metrics optional dependencies.

[ioquatix]

Summary

• Remove metrics and traces from the runtime gemspec dependencies.
• Add metrics and traces to the test/development bundle so provider tests and opt-in provider files still work.

Verification

• ruby -Ilib -e 'spec = Gem::Specification.load("async.gemspec"); abort("unexpected dependency") if spec.dependencies.any? {|dependency| ["metrics", "traces"].include?(dependency.name)}; require "async"; loaded = .grep(%r{/(metrics|traces)/|/(metrics|traces).rb\z}); abort("unexpected load: #{loaded.inspect}") unless loaded.empty?; puts "ok"'
• BUNDLE_GEMFILE="gems.rb" bundle exec ruby -Ilib -e 'require "metrics/provider/async/task"; require "traces/provider/async/task"; require "traces/provider/async/barrier"; puts "ok"'