update metric

Author: shawnm0705

src/features/2.metrics.md

@@ -17,6 +17,7 @@ Metric is used to allow admin configuring custom metrics for an assessment to se
   * `scalar` : can only be linked to a radio button choice question of an assessment
 * `aggregation` defines how to calculate the result. It has the following valid value: (when the `data_type` is `scalar`)
   * `average` : the result will be the average `weight` of the answers for this question
+  * `average_rate` : the result will be the average `weight` of the answers / max `weight` for this question
   * `sum` : the result will be the sum of the `weight` of the answers for this question
   * `count` : the result will be the count of the user that has answered that question
 * `filters` : the filters applied to this metric
@@ -80,11 +81,15 @@ Each combination of data source and data type represents a different metric calc
 </tr>
 
 <tr>
-<td rowspan="5">question</td>
-<td rowspan="5">scalar</td>
+<td rowspan="6">question</td>
+<td rowspan="6">scalar</td>
 <td>average</td>
 <td>The average value of the weight of the question choice</td>
-<td rowspan="4">circle number,<br/>card,<br/>table</td>
+<td rowspan="5">circle number,<br/>card,<br/>table</td>
+</tr>
+<tr>
+<td>average_rate</td>
+<td>The average weight / max weight</td>
 </tr>
 <tr>
 <td>sum</td>