Deal with server not ready gracefully

lib/puma/metrics/app.rb

@@ -6,6 +6,8 @@
 
 module Puma
   module Metrics
+    MetricsNotAvailableError = Class.new(StandardError)
+
     class App
       def initialize(launcher)
         @launcher = launcher
@@ -20,16 +22,36 @@ def call(_env)
           { 'Content-Type' => 'text/plain' },
           [Prometheus::Client::Formats::Text.marshal(Prometheus::Client.registry)]
         ]
+      rescue MetricsNotAvailableError => e
+        [503, { 'Content-Type' => 'text/plain' }, ["#{e.message}\n"]]
       end
 
       def retrieve_and_parse_stats!
-        puma_stats = @launcher.stats
+        puma_stats = fetch_stats
         if puma_stats.is_a?(Hash) # Modern Puma outputs stats as a Symbol-keyed Hash
           @parser.parse(puma_stats)
         else
           @parser.parse(JSON.parse(puma_stats, symbolize_names: true))
         end
       end
+
+      private
+
+      def fetch_stats
+        @launcher.stats
+      rescue NoMethodError
+        # Puma plugins are started in the background along with the server, so
+        # there's a chance that a request will arrive to the server started by
+        # this plugin before the main one has been registered in the launcher.
+        #
+        # If that happens, fetching the stats fails because `@server` is nil,
+        # causing a NoMethodError.
+        #
+        # Ideally this code should detect the case using the launcher public
+        # interface, but no methods expose the information that is required for
+        # that.
+        raise MetricsNotAvailableError, 'Puma is booting up. Stats are not yet ready.'
+      end
     end
   end
 end