[chore] Add test for on startup compaction (#33511)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
While there are
[tests](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/extension/storage/filestorage/client_test.go#L214)
for `OnRebound` compaction, I didn't spot any that cover the `OnStart`
option. This PR adds a unit test for this case.

**Link to tracking Issue:** <Issue number if applicable> ~

**Testing:** <Describe what testing was performed and which tests were
added.> Added

**Documentation:** <Describe the documentation added.> ~

Signed-off-by: ChrsMark <[email protected]>

extension/storage/filestorage/extension_test.go

@@ -17,6 +17,8 @@ import (
 	"go.opentelemetry.io/collector/component/componenttest"
 	"go.opentelemetry.io/collector/extension/experimental/storage"
 	"go.opentelemetry.io/collector/extension/extensiontest"
+	"go.uber.org/zap"
+	"go.uber.org/zap/zaptest/observer"
 )
 
 func TestExtensionIntegrity(t *testing.T) {
@@ -485,3 +487,41 @@ func TestCleanupOnStart(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, 1, len(files))
 }
+
+func TestCompactionOnStart(t *testing.T) {
+	ctx := context.Background()
+
+	logCore, logObserver := observer.New(zap.DebugLevel)
+	logger := zap.New(logCore)
+	set := extensiontest.NewNopSettings()
+	set.Logger = logger
+
+	tempDir := t.TempDir()
+	temp, _ := os.CreateTemp(tempDir, TempDbPrefix)
+	temp.Close()
+
+	f := NewFactory()
+	cfg := f.CreateDefaultConfig().(*Config)
+	cfg.Directory = tempDir
+	cfg.Compaction.Directory = tempDir
+	cfg.Compaction.OnStart = true
+	extension, err := f.CreateExtension(context.Background(), set, cfg)
+	require.NoError(t, err)
+
+	se, ok := extension.(storage.Extension)
+	require.True(t, ok)
+	require.NoError(t, se.Start(ctx, componenttest.NewNopHost()))
+
+	client, err := se.GetClient(
+		ctx,
+		component.KindReceiver,
+		newTestEntity("my_component"),
+		"",
+	)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		// At least one compaction should have happened on start
+		require.GreaterOrEqual(t, len(logObserver.FilterMessage("finished compaction").All()), 1)
+		require.NoError(t, client.Close(context.TODO()))
+	})
+}