Define hit/miss counters for redis diff caching

lib/gitlab/diff/highlight_cache.rb

@@ -17,6 +17,14 @@ module Gitlab
         buckets [100, 1000, 10000, 100000, 1000000, 10000000]
       end
 
+      define_counter :gitlab_redis_diff_caching_hit do
+        docstring 'Redis diff caching hits'
+      end
+
+      define_counter :gitlab_redis_diff_caching_miss do
+        docstring 'Redis diff caching misses'
+      end
+
       def initialize(diff_collection)
         @diff_collection = diff_collection
       end

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -149,5 +149,13 @@ describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
     it 'defines :gitlab_redis_diff_caching_memory_usage_bytes histogram' do
       expect(described_class).to respond_to(:gitlab_redis_diff_caching_memory_usage_bytes)
     end
+
+    it 'defines :gitlab_redis_diff_caching_hit' do
+      expect(described_class).to respond_to(:gitlab_redis_diff_caching_hit)
+    end
+
+    it 'defines :gitlab_redis_diff_caching_miss' do
+      expect(described_class).to respond_to(:gitlab_redis_diff_caching_miss)
+    end
   end
 end