Remove memoization of counters in Gitlab::Gpg

This causes issues when the Prometheus registry is reset. The class may
be holding stale references.

This is not an issue in production but this caused issues in specs
because we clear the registry before Prometheus related specs.

lib/gitlab/gpg.rb

@@ -142,13 +142,11 @@ module Gitlab
     end
 
     def tmp_keychains_created
-      @tmp_keychains_created ||= Gitlab::Metrics.counter(:gpg_tmp_keychains_created_total,
-                                                         'The number of temporary GPG keychains created')
+      Gitlab::Metrics.counter(:gpg_tmp_keychains_created_total, 'The number of temporary GPG keychains created')
     end
 
     def tmp_keychains_removed
-      @tmp_keychains_removed ||= Gitlab::Metrics.counter(:gpg_tmp_keychains_removed_total,
-                                                         'The number of temporary GPG keychains removed')
+      Gitlab::Metrics.counter(:gpg_tmp_keychains_removed_total, 'The number of temporary GPG keychains removed')
     end
   end
 end

spec/lib/gitlab/gpg_spec.rb

@@ -143,11 +143,6 @@ RSpec.describe Gitlab::Gpg do
     end
 
     it 'keeps track of created and removed keychains in counters' do
-      # Gitlab::Gpg may be memoizing stale counters if a preceding spec resets the Prometheus registry
-      # https://gitlab.com/gitlab-org/gitlab/-/issues/286874
-      described_class.remove_instance_variable(:@tmp_keychains_created)
-      described_class.remove_instance_variable(:@tmp_keychains_removed)
-
       created = Gitlab::Metrics.counter(:gpg_tmp_keychains_created_total, 'The number of temporary GPG keychains')
       removed = Gitlab::Metrics.counter(:gpg_tmp_keychains_removed_total, 'The number of temporary GPG keychains')