update doc

Author: shawnm0705

src/features/2.metrics.md

@@ -18,9 +18,10 @@ Metric is used to allow admin configuring custom metrics for an assessment to se
   * `average` : the result will be the average `weight` of the answers 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
-* `filter_type` : the type of filter. Valid value `role` (MVP)
-* `filter_value` : the filter value.
-  * When `filter_type` is `role`, the valid values are `participant, mentor, coordinator, admin`
+* `filters` : the filters applied to this metric
+  * This is a JSON encoded field that contain 2 attributes
+  * `role`: an array of filtered roles. Valid `role` are `participant`, `mentor`, `coordinator`, `admin`
+  * `status`: an array of filtered user status. Valid `status` are `active`, `dropped`
 * `default_calculation_frequency`
 
 ### `analytics_metric_institutions`