fix: lost metrics of flowgraph and write buffer (#42549)

issue: #42548

Signed-off-by: chyezh <chyezh@outlook.com>

pkg/metrics/datanode_metrics.go

@@ -18,6 +18,7 @@ package metrics
 
 import (
 	"fmt"
+	"sync"
 
 	"github.com/prometheus/client_golang/prometheus"
 
@@ -336,8 +337,17 @@ var (
 		}, []string{nodeIDLabelName, "type"})
 )
 
+var registerDNOnce sync.Once
+
 // RegisterDataNode registers DataNode metrics
 func RegisterDataNode(registry *prometheus.Registry) {
+	registerDNOnce.Do(func() {
+		registerDataNodeOnce(registry)
+	})
+}
+
+// registerDataNodeOnce registers DataNode metrics
+func registerDataNodeOnce(registry *prometheus.Registry) {
 	registry.MustRegister(DataNodeNumFlowGraphs)
 	// input related
 	registry.MustRegister(DataNodeConsumeMsgRowsCount)

pkg/metrics/streaming_service_metrics.go

@@ -496,6 +496,10 @@ func RegisterStreamingNode(registry *prometheus.Registry) {
 	registry.MustRegister(StreamingNodeConsumeBytes)
 
 	registerWAL(registry)
+
+	// TODO: after remove the implementation of old data node
+	// Such as flowgraph and writebuffer, we can remove these metrics from streaming node.
+	RegisterDataNode(registry)
 }
 
 // registerWAL registers wal metrics