Adds a map overload for addAttributes

Author: surbhigarg92

gax-java/gax/src/main/java/com/google/api/gax/tracing/MetricsTracer.java

@@ -251,7 +251,7 @@ public void addAttributes(String key, String value) {
    * Gax.
    */
   public void addAttributes(Map<String, String> attributes) {
-    attributes.putAll(attributes);
+    this.attributes.putAll(attributes);
   };
 
   @VisibleForTesting