Define a memory usage histogram for diff caching

Defines a memory usage histogram for eventually tracking the size of
highlighted diffs placed in the redis cache.

lib/gitlab/diff/highlight_cache.rb

@@ -3,6 +3,7 @@
 module Gitlab
   module Diff
     class HighlightCache
+      include Gitlab::Metrics::Methods
       include Gitlab::Utils::StrongMemoize
 
       EXPIRATION = 1.week
@@ -11,6 +12,11 @@ module Gitlab
       delegate :diffable,     to: :@diff_collection
       delegate :diff_options, to: :@diff_collection
 
+      define_histogram :gitlab_redis_diff_caching_memory_usage_bytes do
+        docstring 'Redis diff caching memory usage by key'
+        buckets [100, 1000, 10000, 100000, 1000000, 10000000]
+      end
+
       def initialize(diff_collection)
         @diff_collection = diff_collection
       end

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -154,4 +154,10 @@ describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
       cache.clear
     end
   end
+
+  describe 'metrics' 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
+  end
 end