Fallback to JSON format for '*/*' content types

This change fixes the broken browser metric inspection, as the fallback
content type */* wasn't handled correctly. The normal Prometheus server
- client communication wasn't affected.

lib/prometheus/client/rack/exporter.rb

@@ -20,12 +20,12 @@ def initialize(app, options = {})
           @app = app
           @registry = options[:registry] || Client.registry
           @path = options[:path] || '/metrics'
-          @accpetable = build_dictionary(FORMATS)
+          @acceptable = build_dictionary(FORMATS, FALLBACK)
         end
 
         def call(env)
           if env['PATH_INFO'] == @path
-            format = negotiate(env['HTTP_ACCEPT'], @accpetable, FALLBACK)
+            format = negotiate(env['HTTP_ACCEPT'], @acceptable)
             format ? respond_with(format) : not_acceptable(FORMATS)
           else
             @app.call(env)
@@ -34,8 +34,8 @@ def call(env)
 
         private
 
-        def negotiate(accept, formats, fallback)
-          return fallback if accept.to_s.empty?
+        def negotiate(accept, formats)
+          accept = '*/*' if accept.to_s.empty?
 
           parse(accept).each do |content_type, _|
             return formats[content_type] if formats.key?(content_type)
@@ -81,8 +81,8 @@ def not_acceptable(formats)
           ]
         end
 
-        def build_dictionary(formats)
-          formats.each_with_object({}) do |format, memo|
+        def build_dictionary(formats, fallback)
+          formats.each_with_object('*/*' => fallback) do |format, memo|
             memo[format::CONTENT_TYPE] = format
             memo[format::MEDIA_TYPE] = format
           end

spec/prometheus/client/rack/exporter_spec.rb

@@ -53,10 +53,16 @@
       end
     end
 
-    context 'when client does send a Accept header' do
+    context 'when client does not send a Accept header' do
       include_examples 'ok', {}, json
     end
 
+    context 'when client accpets any media type' do
+      accept = '*/*'
+
+      include_examples 'ok', { 'HTTP_ACCEPT' => accept }, json
+    end
+
     context 'when client requests application/json' do
       accept = 'application/json'
 
@@ -110,5 +116,11 @@
 
       include_examples 'not acceptable', 'HTTP_ACCEPT' => accept
     end
+
+    context 'when client accepts unknown formats and wildcard' do
+      accept = 'fancy/woo;q=0.3, proto/buf;q=0.7, */*;q=0.1'
+
+      include_examples 'ok', { 'HTTP_ACCEPT' => accept }, json
+    end
   end
 end