Enhance Compatibility with Rack 3 and Maintain Support for Rack 2 (#8)

Co-authored-by: Andrey Novikov <envek@envek.name>

lib/yabeda/prometheus/mmap/exporter.rb

@@ -23,7 +23,8 @@ def call(env)
           def start_metrics_server!
             Thread.new do
               default_port = ENV.fetch('PORT', 9394)
-              ::Rack::Handler::WEBrick.run(
+
+              rack_handler.run(
                 rack_app,
                 Host: ENV['PROMETHEUS_EXPORTER_BIND'] || '0.0.0.0',
                 Port: ENV.fetch('PROMETHEUS_EXPORTER_PORT', default_port),
@@ -32,6 +33,18 @@ def start_metrics_server!
             end
           end
 
+          def rack_handler
+            if Gem.loaded_specs['rack']&.version&.>= Gem::Version.new('3.0')
+              require 'rackup'
+              ::Rackup::Handler::WEBrick
+            else
+              ::Rack::Handler::WEBrick
+            end
+          rescue LoadError
+            warn 'Please add gems rackup and webrick to your Gemfile to expose Yabeda metrics from prometheus-mmap'
+            ::Rack::Handler::WEBrick
+          end
+
           def rack_app(exporter = self, path: '/metrics')
             ::Rack::Builder.new do
               use ::Rack::CommonLogger if ENV['PROMETHEUS_EXPORTER_LOG_REQUESTS'] != 'false'