add query metrics throttle

Author: nytian

src/DurableTask.SqlServer.AzureFunctions/SqlMetricsProvider.cs

@@ -3,12 +3,15 @@
 
 namespace DurableTask.SqlServer.AzureFunctions
 {
+    using System;
     using System.Threading;
     using System.Threading.Tasks;
 
     public class SqlMetricsProvider
     {
         readonly SqlOrchestrationService service;
+        DateTime metricsTimeStamp = DateTime.MinValue;
+        SqlScaleMetric? metrics;
 
         public SqlMetricsProvider(SqlOrchestrationService service)
         {
@@ -17,13 +20,22 @@ public SqlMetricsProvider(SqlOrchestrationService service)
 
         public virtual async Task<SqlScaleMetric> GetMetricsAsync(int? previousWorkerCount = null)
         {
-            // GetRecommendedReplicaCountAsync will write a trace if the recommendation results
-            // in a worker count that is different from the worker count we pass in as an argument.
-            int recommendedReplicaCount = await this.service.GetRecommendedReplicaCountAsync(
-                previousWorkerCount,
-                CancellationToken.None);
+            int recommendedReplicaCount;
 
-            return new SqlScaleMetric { RecommendedReplicaCount = recommendedReplicaCount };
+            // We only want to query the metrics every 5 seconds.
+            if (this.metrics == null || DateTime.UtcNow >= this.metricsTimeStamp.AddSeconds(5))
+            {
+                // GetRecommendedReplicaCountAsync will write a trace if the recommendation results
+                // in a worker count that is different from the worker count we pass in as an argument.
+                recommendedReplicaCount = await this.service.GetRecommendedReplicaCountAsync(
+                    previousWorkerCount,
+                    CancellationToken.None);
+
+                this.metricsTimeStamp = DateTime.UtcNow;
+                this.metrics = new SqlScaleMetric { RecommendedReplicaCount = recommendedReplicaCount };
+            }
+
+            return this.metrics;
         }
     }
 }