Implement ToRowser interface for DistinctTimestamp

Because the ToRowser interface was not implemented for DistinctTimestamp, there was
a error when using the GRPC endpoint to call Distinct(All(), field=ts). Implementing
the ToRowser interface for DistinctTimestamp solves that problem.

Related to SUP-210: WebUI, Python - Distinct() does not work for Timestamp field

executor.go

@@ -1587,6 +1587,34 @@ type DistinctTimestamp struct {
 	Name   string
 }
 
+var _ proto.ToRowser = DistinctTimestamp{}
+
+// ToRows implements the ToRowser interface.
+func (d DistinctTimestamp) ToRows(callback func(*proto.RowResponse) error) error {
+	for _, ts := range d.Values {
+		row := &proto.RowResponse{
+			Headers: []*proto.ColumnInfo{
+				{
+					Name:     d.Name,
+					Datatype: "timestamp",
+				},
+			},
+			Columns: []*proto.ColumnResponse{
+				{
+					ColumnVal: &proto.ColumnResponse_TimestampVal{
+						TimestampVal: ts,
+					},
+				},
+			},
+		}
+		if err := callback(row); err != nil {
+			return errors.Wrap(err, "calling callback")
+		}
+	}
+
+	return nil
+}
+
 // Union returns the union of the values of `d` and `other`
 func (d *DistinctTimestamp) Union(other DistinctTimestamp) DistinctTimestamp {
 	both := map[string]string{}

executor_test.go

@@ -8546,3 +8546,66 @@ func MinMaxTimestampNodeTester(t *testing.T, numNodes int) {
 		t.Fatalf("incorrect max timestamp val. expected: %v, got %v\n", expected, min)
 	}
 }
+
+// DistinctTimestamp ToRows should properly encode the timestamp
+func TestDistinctTimestampToRows(t *testing.T) {
+	d := pilosa.DistinctTimestamp{
+		Values: []string{
+			"2022-03-24T12:08:37Z",
+			"2022-03-24T12:08:47Z",
+			"2022-03-24T12:08:57Z",
+		},
+		Name: "timestamp",
+	}
+
+	expectedHeaders := []*proto.ColumnInfo{
+		{
+			Name:     d.Name,
+			Datatype: "timestamp",
+		},
+	}
+	expected := []*proto.RowResponse{
+		{
+			Headers: expectedHeaders,
+			Columns: []*proto.ColumnResponse{
+				{
+					ColumnVal: &proto.ColumnResponse_TimestampVal{
+						TimestampVal: "2022-03-24T12:08:37Z",
+					},
+				},
+			},
+		},
+		{
+			Headers: expectedHeaders,
+			Columns: []*proto.ColumnResponse{
+				{
+					ColumnVal: &proto.ColumnResponse_TimestampVal{
+						TimestampVal: "2022-03-24T12:08:47Z",
+					},
+				},
+			},
+		},
+		{
+			Headers: expectedHeaders,
+			Columns: []*proto.ColumnResponse{
+				{
+					ColumnVal: &proto.ColumnResponse_TimestampVal{
+						TimestampVal: "2022-03-24T12:08:57Z",
+					},
+				},
+			},
+		},
+	}
+
+	rows := []*proto.RowResponse{}
+	d.ToRows(func(r *proto.RowResponse) error {
+		rows = append(rows, r)
+		return nil
+	})
+
+	for i, row := range rows {
+		if !reflect.DeepEqual(row, expected[i]) {
+			t.Errorf("expected %v, got %v", expected[i], row)
+		}
+	}
+}