Add group by support to count API (#303)

Author: rodrigozhou

temporal/api/workflowservice/v1/request_response.proto

@@ -781,7 +781,21 @@ message CountWorkflowExecutionsRequest {
 }
 
 message CountWorkflowExecutionsResponse {
+    // If `query` is not grouping by any field, the count is an approximate number
+    // of workflows that matches the query.
+    // If `query` is grouping by a field, the count is simply the sum of the counts
+    // of the groups returned in the response. This number can be smaller than the
+    // total number of workflows matching the query.
     int64 count = 1;
+
+    // `groups` contains the groups if the request is grouping by a field.
+    // The list might not be complete, and the counts of each group is approximate.
+    repeated AggregationGroup groups = 2;
+
+    message AggregationGroup {
+        repeated temporal.api.common.v1.Payload group_values = 1;
+        int64 count = 2;
+    }
 }
 
 message GetSearchAttributesRequest {