Allow use of Hash-shaped stats

Future puma versions will be outputting a Hash instead
of a string of JSON, so the parsing step is no longer needed.

Also update key use to always use symbols, since this allows
better object reuse (and the builtin #stats method will be
returning Symbol-keyed hashes natively)

lib/puma/metrics/app.rb

@@ -14,13 +14,22 @@ def initialize(launcher)
       end
 
       def call(_env)
-        @parser.parse JSON.parse(@launcher.stats)
+        retrieve_and_parse_stats!
         [
           200,
           { 'Content-Type' => 'text/plain' },
           [Prometheus::Client::Formats::Text.marshal(Prometheus::Client.registry)]
         ]
       end
+
+      def retrieve_and_parse_stats!
+        puma_stats = @launcher.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
     end
   end
 end

lib/puma/metrics/parser.rb

@@ -10,10 +10,10 @@ def initialize(clustered = false)
         register_clustered_metrics if clustered
       end
 
-      def parse(stats, labels = {})
-        stats.each do |key, value|
-          value.each { |s| parse(s, labels.merge(index: s['index'])) } if key == 'worker_status'
-          parse(value, labels) if key == 'last_status'
+      def parse(symbol_keyed_stats, labels = {})
+        symbol_keyed_stats.each do |key, value|
+          value.each { |s| parse(s, labels.merge(index: s[:index])) } if key == :worker_status
+          parse(value, labels) if key == :last_status
           update_metric(key, value, labels)
         end
       end