Fix order-dependent Prometheus metrics specs

https://gitlab.com/gitlab-org/gitlab/merge_requests/22355 modified a
class instance variable, causing the error state to be enabled for all
subsequent tests. This commit fixes that by clearing the errors after
the tests run.

Closes https://gitlab.com/gitlab-org/gitlab/issues/196048

lib/gitlab/metrics/prometheus.rb

@@ -62,10 +62,18 @@ module Gitlab
         end
 
         def error_detected!
+          set_error!(true)
+        end
+
+        def clear_errors!
+          set_error!(false)
+        end
+
+        def set_error!(status)
           clear_memoization(:prometheus_metrics_enabled)
 
           PROVIDER_MUTEX.synchronize do
-            @error = true
+            @error = status
           end
         end
 

spec/lib/gitlab/metrics/prometheus_spec.rb

@@ -6,6 +6,10 @@ describe Gitlab::Metrics::Prometheus, :prometheus do
   let(:all_metrics) { Gitlab::Metrics }
   let(:registry) { all_metrics.registry }
 
+  after do
+    all_metrics.clear_errors!
+  end
+
   describe '#reset_registry!' do
     it 'clears existing metrics' do
       registry.counter(:test, 'test metric')