fix(rds): switch connection monitor to average (#233)

Fixes #231 

---

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Author: relm923

lib/monitoring/aws-rds/RdsClusterMetricFactory.ts

@@ -54,7 +54,7 @@ export class RdsClusterMetricFactory {
   metricTotalConnectionCount() {
     return this.metric(
       "DatabaseConnections",
-      MetricStatistic.SUM,
+      MetricStatistic.AVERAGE,
       "Connections"
     );
   }