update connectionStateListener metrics update (Azure#28375)

Co-authored-by: annie-mac <annie-mac@annie-macs-MacBook-Pro.local>

Author: xinlian12

sdk/cosmos/azure-cosmos/src/main/java/com/azure/cosmos/implementation/directconnectivity/rntbd/RntbdConnectionStateListenerMetrics.java

@@ -46,9 +46,10 @@ public RntbdConnectionStateListenerMetricsJsonSerializer() {
         public void serialize(RntbdConnectionStateListenerMetrics metrics, JsonGenerator writer, SerializerProvider serializers) throws IOException {
             writer.writeStartObject();
 
-            writer.writeStringField(
-                "lastCallTimestamp",
-                metrics.lastCallTimestamp.get() == null ? "N/A" : metrics.lastCallTimestamp.toString());
+            if (metrics.lastCallTimestamp.get() != null) {
+                writer.writeStringField(
+                        "lastCallTimestamp", metrics.lastCallTimestamp.toString());
+            }
 
             if (metrics.lastActionableContext.get() != null) {
                 writer.writeStringField("lastActionableContext", metrics.lastActionableContext.get().toString());

sdk/cosmos/azure-cosmos/src/test/java/com/azure/cosmos/CosmosDiagnosticsTest.java

@@ -934,7 +934,7 @@ private void validateRntbdStatistics(CosmosDiagnostics cosmosDiagnostics,
         if (connectionStateListenerEnabled) {
 
             assertThat(connectionStateListenerMetrics).isNotNull();
-            assertThat(connectionStateListenerMetrics.get("lastCallTimestamp")).isEqualTo("N/A");
+            assertThat(connectionStateListenerMetrics.get("lastCallTimestamp")).isNull();
             assertThat(connectionStateListenerMetrics.get("lastActionableContext")).isNull();
         } else {
             assertThat(connectionStateListenerMetrics).isNull();