refine grafana metrics for replica selection (#25319)

Signed-off-by: Wei Liu <wei.liu@zilliz.com>

internal/proxy/look_aside_balancer.go

@@ -102,7 +102,7 @@ func (b *LookAsideBalancer) SelectNode(ctx context.Context, availableNodes []int
 		}
 
 		score := b.calculateScore(cost, executingNQ.Load())
-		metrics.ProxyWorkLoadScore.WithLabelValues(strconv.FormatInt(paramtable.GetNodeID(), 10)).Observe(score)
+		metrics.ProxyWorkLoadScore.WithLabelValues(strconv.FormatInt(node, 10)).Set(score)
 
 		if targetNode == -1 || score < targetScore {
 			targetScore = score

pkg/metrics/proxy_metrics.go

@@ -249,14 +249,15 @@ var (
 		}, []string{usernameLabelName})
 
 	// ProxyWorkLoadScore record the score that measured query node's workload.
-	ProxyWorkLoadScore = prometheus.NewHistogramVec(
-		prometheus.HistogramOpts{
+	ProxyWorkLoadScore = prometheus.NewGaugeVec(
+		prometheus.GaugeOpts{
 			Namespace: milvusNamespace,
 			Subsystem: typeutil.ProxyRole,
 			Name:      "workload_score",
 			Help:      "score that measured query node's workload",
-			Buckets:   buckets,
-		}, []string{nodeIDLabelName})
+		}, []string{
+			nodeIDLabelName,
+		})
 )
 
 // RegisterProxy registers Proxy metrics