add fillmode "last" to sql datasource

This adds a new fill mode last (last observation carried forward) for grafana
to the sql datasources. This fill mode will fill in the last seen value in a
series when a timepoint is missing or NULL if no value for that series has
been seen yet.

pkg/tsdb/postgres/macros.go

@@ -116,9 +116,13 @@ func (m *postgresMacroEngine) evaluateMacro(name string, args []string) (string,
 		if len(args) == 3 {
 			m.query.Model.Set("fill", true)
 			m.query.Model.Set("fillInterval", interval.Seconds())
-			if args[2] == "NULL" {
-				m.query.Model.Set("fillNull", true)
-			} else {
+			switch args[2] {
+			case "NULL":
+				m.query.Model.Set("fillMode", "null")
+			case "last":
+				m.query.Model.Set("fillMode", "last")
+			default:
+				m.query.Model.Set("fillMode", "value")
 				floatVal, err := strconv.ParseFloat(args[2], 64)
 				if err != nil {
 					return "", fmt.Errorf("error parsing fill value %v", args[2])

pkg/tsdb/postgres/postgres_test.go

@@ -276,7 +276,7 @@ func TestPostgres(t *testing.T) {
 
 			})
 
-			Convey("When doing a metric query using timeGroup with float fill enabled", func() {
+			Convey("When doing a metric query using timeGroup with value fill enabled", func() {
 				query := &tsdb.TsdbQuery{
 					Queries: []*tsdb.Query{
 						{
@@ -303,6 +303,34 @@ func TestPostgres(t *testing.T) {
 			})
 		})
 
+		Convey("When doing a metric query using timeGroup with last fill enabled", func() {
+			query := &tsdb.TsdbQuery{
+				Queries: []*tsdb.Query{
+					{
+						Model: simplejson.NewFromAny(map[string]interface{}{
+							"rawSql": "SELECT $__timeGroup(time, '5m', last), avg(value) as value FROM metric GROUP BY 1 ORDER BY 1",
+							"format": "time_series",
+						}),
+						RefId: "A",
+					},
+				},
+				TimeRange: &tsdb.TimeRange{
+					From: fmt.Sprintf("%v", fromStart.Unix()*1000),
+					To:   fmt.Sprintf("%v", fromStart.Add(34*time.Minute).Unix()*1000),
+				},
+			}
+
+			resp, err := endpoint.Query(nil, nil, query)
+			So(err, ShouldBeNil)
+			queryResult := resp.Results["A"]
+			So(queryResult.Error, ShouldBeNil)
+
+			points := queryResult.Series[0].Points
+			So(points[2][0].Float64, ShouldEqual, 15.0)
+			So(points[3][0].Float64, ShouldEqual, 15.0)
+			So(points[6][0].Float64, ShouldEqual, 20.0)
+		})
+
 		Convey("Given a table with metrics having multiple values and measurements", func() {
 			type metric_values struct {
 				Time                time.Time