add tooltip

public/app/plugins/datasource/cloudwatch/partials/config.html

@@ -43,7 +43,7 @@
       Custom Metrics Attributes
     </li>
     <li>
-      <input type="text" class="tight-form-input input-xlarge" ng-model='current.jsonData.customMetricsAttributes[0]' ng-init="current.jsonData.customMetricsAttributes = current.jsonData.customMetricsAttributes || []" placeholder=""></input>
+      <input type="text" class="tight-form-input input-xlarge" ng-model='current.jsonData.customMetricsAttributes[0]' ng-init="current.jsonData.customMetricsAttributes = current.jsonData.customMetricsAttributes || []" placeholder="JSON url" bs-tooltip="'Set JSON url of the result, \'aws cloudwatch list-metrics --output json\''"></input>
     </li>
   </ul>
   <div class="clearfix"></div>