update doc

Author: shawnm0705

src/features/2.metrics.md

@@ -64,6 +64,31 @@ The table to store the metric report records
 * `count` : how many data have we gone through to do the calculation
 * `created` : date when the record gets calculated
 
+## Metric Types
+
+<table>
+<tr>
+<th>data_source</th>
+<th>data_type</th>
+<th>aggregation</th>
+<th>description</th>
+</tr>
+<tr>
+<td rowspan="3">question</td>
+<td rowspan="3">scalar</td>
+<td>average</td>
+<td>The average value of the weight of the question choice</td>
+</tr>
+<tr>
+<td>sum</td>
+<td>The sum value of the weight of the question choice</td>
+</tr>
+<tr>
+<td>count</td>
+<td>Count how many users have answered this question</td>
+</tr>
+</table>
+
 ## Workflow
 
 ### Create metric