[skip-ci]Add comment in data sync service (#8002)

Signed-off-by: Nyangxuan <xuan.yang@zilliz.com>

internal/datanode/data_node.go

@@ -578,6 +578,7 @@ func (node *DataNode) GetStatisticsChannel(ctx context.Context) (*milvuspb.Strin
 	}, nil
 }
 
+// GetMetrics return datanode metrics
 // TODO(dragondriver): cache the Metrics and set a retention to the cache
 func (node *DataNode) GetMetrics(ctx context.Context, req *milvuspb.GetMetricsRequest) (*milvuspb.GetMetricsResponse, error) {
 	log.Debug("DataNode.GetMetrics",

internal/datanode/data_sync_service.go

@@ -27,6 +27,7 @@ import (
 	"go.uber.org/zap"
 )
 
+// dataSyncService controls a flowgraph for a specific collection
 type dataSyncService struct {
 	ctx          context.Context
 	cancelFn     context.CancelFunc
@@ -78,6 +79,7 @@ func newDataSyncService(ctx context.Context,
 	return service, nil
 }
 
+// start starts the flowgraph in datasyncservice
 func (dsService *dataSyncService) start() {
 	if dsService.fg != nil {
 		log.Debug("Data Sync Service starting flowgraph")
@@ -96,6 +98,7 @@ func (dsService *dataSyncService) close() {
 	dsService.cancelFn()
 }
 
+// initNodes inits a TimetickedFlowGraph
 func (dsService *dataSyncService) initNodes(vchanInfo *datapb.VchannelInfo) error {
 	// TODO: add delete pipeline support
 	dsService.fg = flowgraph.NewTimeTickedFlowGraph(dsService.ctx)