Make traces and metrics optional

[ioquatix]

Summary

• Remove direct runtime dependencies on metrics and traces.
• Keep metrics and traces available for the test/development bundle.
• Require traces/provider explicitly from trace provider files.
• Add an Unreleased release note.

Testing

• bundle exec ruby -e 'require "async/http"'
• bundle exec ruby -e 'require "metrics/provider/async/http"'
• bundle exec ruby -e 'require "traces/provider/async/http"; require "traces/provider/async/http/protocol/http1/client"; require "traces/provider/async/http/protocol/http2/client"'
• bundle exec rubocop async-http.gemspec gems.rb lib/traces/provider/async/http/client.rb lib/traces/provider/async/http/server.rb lib/traces/provider/async/http/protocol/http1/client.rb lib/traces/provider/async/http/protocol/http2/client.rb
• bundle exec sus \test/async/http/body.rb
  test/async/http/body/hijack.rb
  test/async/http/body/pipe.rb
  test/async/http/client.rb
  test/async/http/client/google.rb
  test/async/http/endpoint.rb
  test/async/http/internet.rb
  test/async/http/internet/instance.rb
  test/async/http/middleware/location_redirector.rb
  test/async/http/mock.rb
  test/async/http/protocol/http.rb
  test/async/http/protocol/http1.rb
  test/async/http/protocol/http10.rb
  test/async/http/protocol/http11.rb
  test/async/http/protocol/http11/desync.rb
  test/async/http/protocol/http11/invalid_trailer.rb
  test/async/http/protocol/http2.rb
  test/async/http/protocol/http2/connection_close_with_active_streams.rb
  test/async/http/protocol/http2/invalid_trailer.rb
  test/async/http/protocol/http2/request_failed_on_refused_stream.rb
  test/async/http/protocol/http2/response_close_on_stream_error.rb
  test/async/http/protocol/http2/rst_stream_mid_write.rb
  test/async/http/protocol/https.rb
  test/async/http/proxy.rb
  test/async/http/retry.rb
  test/async/http/server.rb
  test/async/http/ssl.rb
  test/async/http/statistics.rb
  test/protocol/http/body/stream.rb
  test/protocol/http/body/streamable.rb
  test/rack/test.rb

Note: A plain bundle exec sus also picks up an unrelated untracked file, test/async/http/protocol/http2/put_reset_before_response.rb, which currently errors and is not included in this PR.