Begin observing size of cached highlighted diffs

lib/gitlab/diff/highlight_cache.rb

@@ -101,6 +101,8 @@ module Gitlab
             #
             redis.expire(key, EXPIRATION)
           end
+
+          record_memory_usage(fetch_memory_usage(redis, key))
         end
 
         # Subsequent read_file calls would need the latest cache.
@@ -109,6 +111,23 @@ module Gitlab
         clear_memoization(:cacheable_files)
       end
 
+      def record_memory_usage(memory_usage)
+        if memory_usage
+          self.class.gitlab_redis_diff_caching_memory_usage_bytes.observe({}, memory_usage)
+        end
+      end
+
+      def fetch_memory_usage(redis, key)
+        # Redis versions prior to 4.0.0 do not support memory usage reporting
+        #   for a specific key. As of 11-March-2020 we support Redis 3.x, so
+        #   need to account for this. We can remove this check once we
+        #   officially cease supporting versions <4.0.0.
+        #
+        return if Gem::Version.new(redis.info["redis_version"]) < Gem::Version.new("4")
+
+        redis.memory("USAGE", key)
+      end
+
       def file_paths
         strong_memoize(:file_paths) do
           diff_files.collect(&:file_path)

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -97,6 +97,28 @@ 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 if Redis version >= 4' do
+      allow_next_instance_of(Redis) do |redis|
+        allow(redis).to receive(:info).and_return({ "redis_version" => "4.0.0" })
+
+        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
+    end
+
+    it 'does not update memory usage metrics if Redis version < 4' do
+      allow_next_instance_of(Redis) do |redis|
+        allow(redis).to receive(:info).and_return({ "redis_version" => "3.0.0" })
+
+        expect(described_class.gitlab_redis_diff_caching_memory_usage_bytes)
+          .not_to receive(:observe).and_call_original
+
+        cache.send(:write_to_redis_hash, diff_hash)
+      end
+    end
+
     context 'different diff_collections for the same diffable' do
       before do
         cache.write_if_empty