MQTT Username concatenate metrics query parameters (#264)

Co-authored-by: Leonardo Nodari <[email protected]>

Author: bretambrose

sdk/src/main/java/software/amazon/awssdk/iot/AwsIotMqttConnectionBuilder.java

@@ -542,7 +542,11 @@ public MqttClientConnection build() {
             if (connectionConfig.getUsername() != null) {
                 usernameOrEmpty = connectionConfig.getUsername();
             }
-            connectionConfig.setUsername(String.format("%s?SDK=JavaV2&Version=%s", usernameOrEmpty, new PackageInfo().version.toString()));
+            String queryStringConcatenation = "?";
+            if (usernameOrEmpty.contains("?")) {
+                queryStringConcatenation = "&";
+            }
+            connectionConfig.setUsername(String.format("%s%sSDK=JavaV2&Version=%s", usernameOrEmpty, queryStringConcatenation, new PackageInfo().version.toString()));
 
             if (connectionConfig.getUseWebsockets() && connectionConfig.getWebsocketHandshakeTransform() == null) {
                 if (websocketCredentialsProvider == null) {