Stream the Diff Stats back from Gitaly

This was causing an issue with the timing shown on the performance
  bar for this gitaly call. It was only measuring the time to make
  the call, and not the time taken to stream it back.

Issue: gitlab.com/gitlab-org/gitlab/-/issues/209786
Merge Request: gitlab.com/gitlab-org/gitlab/-/merge_requests/33606

lib/gitlab/gitaly_client/commit_service.rb

@@ -212,8 +212,9 @@ module Gitlab
           right_commit_id: right_commit_sha
         )
 
-        response = GitalyClient.call(@repository.storage, :diff_service, :diff_stats, request, timeout: GitalyClient.medium_timeout)
-        response.flat_map(&:stats)
+        GitalyClient.streaming_call(@repository.storage, :diff_service, :diff_stats, request, timeout: GitalyClient.medium_timeout) do |response|
+          response.flat_map(&:stats)
+        end
       end
 
       def find_all_commits(opts = {})

spec/lib/gitlab/gitaly_client/commit_service_spec.rb

@@ -123,16 +123,20 @@ describe Gitlab::GitalyClient::CommitService do
   describe '#diff_stats' do
     let(:left_commit_id) { 'master' }
     let(:right_commit_id) { 'cfe32cf61b73a0d5e9f13e774abde7ff789b1660' }
+    let(:response_value) { [double(stats: stat)] }
+    let(:stat) { double }
 
-    it 'sends an RPC request' do
+    it 'sends an RPC request and returns the stats' do
       request = Gitaly::DiffStatsRequest.new(repository: repository_message,
                                              left_commit_id: left_commit_id,
                                              right_commit_id: right_commit_id)
 
       expect_any_instance_of(Gitaly::DiffService::Stub).to receive(:diff_stats)
-        .with(request, kind_of(Hash)).and_return([])
+        .with(request, kind_of(Hash)).and_return(response_value)
+
+      returned_value = described_class.new(repository).diff_stats(left_commit_id, right_commit_id)
 
-      described_class.new(repository).diff_stats(left_commit_id, right_commit_id)
+      expect(returned_value).to eq([stat])
     end
   end