feat(alerting): add support for count reducer

pkg/services/alerting/conditions/reducer.go

@@ -42,6 +42,8 @@ func (s *SimpleReducer) Reduce(series *tsdb.TimeSeries) float64 {
 	case "mean":
 		meanPosition := int64(len(series.Points) / 2)
 		value = series.Points[meanPosition][0]
+	case "count":
+		value = float64(len(series.Points))
 	}
 
 	return value

pkg/services/alerting/conditions/reducer_test.go

@@ -34,6 +34,10 @@ func TestSimpleReducer(t *testing.T) {
 			So(result, ShouldEqual, float64(2))
 		})
 
+		Convey("count", func() {
+			result := testReducer("count", 1, 2, 3000)
+			So(result, ShouldEqual, float64(3))
+		})
 	})
 }