Fixes a panic in dry-run when using external labels. (#2075)

Fixes #1972.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

pkg/util/flagext/labelset.go

@@ -7,6 +7,9 @@ import (
 	"strings"
 
 	"github.com/prometheus/common/model"
+	"gopkg.in/yaml.v2"
+
+	"github.com/grafana/loki/pkg/util"
 )
 
 // LabelSet is a labelSet that can be used as a flag.
@@ -78,3 +81,12 @@ func (v *LabelSet) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	v.LabelSet = lbSet
 	return nil
 }
+
+// MarshalYAML implements yaml.Marshaller.
+func (v LabelSet) MarshalYAML() (interface{}, error) {
+	out, err := yaml.Marshal(util.ModelLabelSetToMap(v.LabelSet))
+	if err != nil {
+		return nil, err
+	}
+	return string(out), nil
+}