linguist: Add benchmark for languageStatstoon-linguist-cleanup

I was curious how much extra time it would take to calculate the
FileCount:

    goos: linux
    goarch: amd64
    pkg: gitlab.com/gitlab-org/gitaly/v15/internal/gitaly/linguist
    cpu: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
    BenchmarkLanguageStats
    BenchmarkLanguageStats/totals
    BenchmarkLanguageStats/totals-8         1       4298347856 ns/op
    BenchmarkLanguageStats/allCounts
    BenchmarkLanguageStats/allCounts-8      1       6809731635 ns/op

In this bench it takes about 4 seconds to load the stats from file, and
we would calculate the FileCount it takes about 7 seconds in total.

1 files changed, 55 insertions, 0 deletions

diff --git a/internal/gitaly/linguist/language_stats_test.go b/internal/gitaly/linguist/language_stats_test.go
index 69a2c3b4f..a2a0d8779 100644
--- a/internal/gitaly/linguist/language_stats_test.go
+++ b/internal/gitaly/linguist/language_stats_test.go
@@ -252,3 +252,58 @@ func TestLanguageStats_save(t *testing.T) {
 	require.Equal(t, s.Totals, loaded.Totals)
 	require.Equal(t, s.ByFile, loaded.ByFile)
 }
+
+func BenchmarkLanguageStats(b *testing.B) {
+	languages := []string{
+		"Ruby",
+		"Javascript",
+		"C++",
+		"Golang",
+		"HTML",
+		"CSS",
+		"SQL",
+		"Assembly",
+		"Elixir",
+		"C#",
+		"Kotlin",
+		"Zig",
+	}
+
+	stats := newLanguageStats()
+	lenLang := len(languages)
+
+	ctx := testhelper.Context(b)
+	cfg := testcfg.Build(b)
+	repoProto, _ := gittest.CreateRepository(b, ctx, cfg, gittest.CreateRepositoryConfig{
+		SkipCreationViaService: true,
+	})
+
+	repo := localrepo.NewTestRepo(b, cfg, repoProto)
+
+	rand.Seed(time.Now().UnixNano())
+
+	for i := 0; i < 3_000_000; i++ {
+		stats.add(
+			fmt.Sprintf("file_%010d", i),
+			languages[rand.Intn(lenLang-1)],
+			uint64(rand.Int()),
+		)
+	}
+
+	require.NoError(b, stats.save(repo, "1337C0DE"))
+
+	b.Run("totals", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_, _ = initLanguageStats(repo)
+			_ = stats.Totals
+		}
+	})
+
+	b.Run("allCounts", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_, err := initLanguageStats(repo)
+			require.NoError(b, err)
+			_ = stats.allCounts()
+		}
+	})
+}