Support Rack 3

[bibendi]

Hey! 🙂

#<Thread:0x0000000121ff6708 /Users/bibendi/.asdf/installs/ruby/3.3.5/lib/ruby/gems/3.3.0/gems/yabeda-prometheus-mmap-0.4.0/lib/yabeda/prometheus/mmap/exporter.rb:24 run> terminated with exception (report_on_exception is true):
/Users/bibendi/.asdf/installs/ruby/3.3.5/lib/ruby/gems/3.3.0/gems/yabeda-prometheus-mmap-0.4.0/lib/yabeda/prometheus/mmap/exporter.rb:39:in `rack_handler': uninitialized constant Rackup::Handler::WEBrick (NameError)

              ::Rackup::Handler::WEBrick
                               ^^^^^^^^^
	from /Users/bibendi/.asdf/installs/ruby/3.3.5/lib/ruby/gems/3.3.0/gems/yabeda-prometheus-mmap-0.4.0/lib/yabeda/prometheus/mmap/exporter.rb:27:in `block in start_metrics_server!'

[geoffharcourt]

This was accomplished in 9d84af2!