rbd: add debug logging to ProcessMetadata for CBT diagnostics

Add debug log statements to ProcessMetadata to log the snapshot ID,
volume size, and DiffIterateByID configuration parameters. This
helps diagnose cases where GetMetadataAllocated returns empty
streams despite the underlying rbd diff working correctly.

Ref: kaovilai/cephcsi-cbt-e2e#2

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people

internal/rbd/snap_diff.go

@@ -58,6 +58,8 @@ func (rbdSnap *rbdSnapshot) ProcessMetadata(
 		return fmt.Errorf("failed to get snapshot ID of %q: %w", rbdSnap, err)
 	}
 
+	log.DebugLog(ctx, "ProcessMetadata: image=%q, snapID=%d, VolSize=%d", rbdSnap, snapID, rbdSnap.VolSize)
+
 	err = image.SetSnapByID(snapID)
 	if err != nil {
 		return fmt.Errorf("failed to set snapshot %q by ID %d: %w",
@@ -107,12 +109,17 @@ func (rbdSnap *rbdSnapshot) ProcessMetadata(
 		diffIterateByIDConfig.FromSnapID = fromSnapID
 	}
 
+	log.DebugLog(ctx, "ProcessMetadata: DiffIterateByID config: Offset=%d, Length=%d, FromSnapID=%d",
+		diffIterateByIDConfig.Offset, diffIterateByIDConfig.Length, diffIterateByIDConfig.FromSnapID)
+
 	diffIterateErr := image.DiffIterateByID(diffIterateByIDConfig)
 	err = handleDiffIterateError(diffIterateErr)
 	if err != nil {
 		return fmt.Errorf("failed to get diff: %w", err)
 	}
 
+	log.DebugLog(ctx, "ProcessMetadata: DiffIterateByID completed, remaining blocks=%d", len(changedBlocks))
+
 	if len(changedBlocks) != 0 {
 		// Send any remaining changed blocks after the loop.
 		err = sendResponse(changedBlocks)