INFRA-25589: test fix Content-Type header spec for Prometheus exposition format 0.0.4

Update rack_app_spec to assert the correct `version=0.0.4` parameter
in the Content-Type header. Add dedicated assertion in
metrics_controller_spec since that is where the header is set.
Tighten CHANGELOG entry wording.

Fixes INFRA-25589
Co-Authored-By: Claude <noreply@anthropic.com>

Author: bc-dhruvin

CHANGELOG.md

@@ -4,7 +4,7 @@ Changelog for the bc-prometheus-ruby gem.
 
 ## 0.8.2
 
-- Fix Prometheus 3.x Content-Type compliance: add `version=0.0.4` qualifier to the `text/plain` Content-Type header served by the Puma metrics controller (INFRA-25589)
+- Add `version=0.0.4` to `Content-Type` header for Prometheus exposition format 0.0.4 compliance (INFRA-25589)
 
 ## 0.8.1
 

spec/bigcommerce/prometheus/servers/puma/controllers/metrics_controller_spec.rb

@@ -49,6 +49,10 @@
 ruby_collector_sessions_total 19'
     end
 
+    it 'sets Prometheus text format 0.0.4 content type' do
+      expect(subject.headers['Content-Type']).to eq('text/plain; version=0.0.4; charset=utf-8')
+    end
+
     context 'when the metrics fail to parse' do
       let(:timeout_exception) { ::Timeout::Error }
       before do

spec/bigcommerce/prometheus/servers/puma/rack_app_spec.rb

@@ -41,9 +41,9 @@
         }
       end
 
-      it 'returns content type of text/plain' do
+      it 'returns Prometheus text format 0.0.4 content type' do
         _status, headers, _body = subject
-        expect(headers['Content-Type']).to eq('text/plain; charset=utf-8')
+        expect(headers['Content-Type']).to eq('text/plain; version=0.0.4; charset=utf-8')
       end
     end