Begin observing size of cached highlighted diffs

lib/gitlab/diff/highlight_cache.rb

@@ -93,6 +93,8 @@ module Gitlab
             #
             redis.expire(key, EXPIRATION)
           end
+
+          record_metrics(redis.memory("USAGE", key))
         end
 
         # Subsequent read_file calls would need the latest cache.
@@ -101,6 +103,10 @@ module Gitlab
         clear_memoization(:cacheable_files)
       end
 
+      def record_metrics(memory_usage)
+        self.class.gitlab_redis_diff_caching_memory_usage_bytes.observe({}, memory_usage)
+      end
+
       def file_paths
         strong_memoize(:file_paths) do
           diff_files.collect(&:file_path)

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -101,6 +101,13 @@ describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
       let(:paths) { merge_request.diffs.raw_diff_files.select(&:text?).map(&:file_path) }
     end
 
+    it 'updates memory usage metrics' do
+      expect(described_class.gitlab_redis_diff_caching_memory_usage_bytes)
+        .to receive(:observe).and_call_original
+
+      cache.send(:write_to_redis_hash, diff_hash)
+    end
+
     context 'different diff_collections for the same diffable' do
       before do
         cache.write_if_empty