relabel: blank replacement deletes label post-regexp

If `cfg.TargetLabel` is a template like `$1`, it won't match any labels,
so no point calling `lb.Del` with it.

Similarly if `target` is not a valid label name, it won't match any
labels, so don't call with that either.

The intention seems to have been that a blank _value_ would delete the
target, so change that code to use `target` instead of `cfg.TargetLabel`.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

model/relabel/relabel.go

@@ -274,12 +274,11 @@ func relabel(cfg *Config, lb *labels.Builder) (keep bool) {
 		}
 		target := model.LabelName(cfg.Regex.ExpandString([]byte{}, cfg.TargetLabel, val, indexes))
 		if !target.IsValid() {
-			lb.Del(cfg.TargetLabel)
 			break
 		}
 		res := cfg.Regex.ExpandString([]byte{}, cfg.Replacement, val, indexes)
 		if len(res) == 0 {
-			lb.Del(cfg.TargetLabel)
+			lb.Del(string(target))
 			break
 		}
 		lb.Set(string(target), string(res))

model/relabel/relabel_test.go

@@ -214,6 +214,25 @@ func TestRelabel(t *testing.T) {
 				"a": "boo",
 			}),
 		},
+		{
+			// Blank replacement should delete the label.
+			input: labels.FromMap(map[string]string{
+				"a": "foo",
+				"f": "baz",
+			}),
+			relabel: []*Config{
+				{
+					SourceLabels: model.LabelNames{"a"},
+					Regex:        MustNewRegexp("(f).*"),
+					TargetLabel:  "$1",
+					Replacement:  "$2",
+					Action:       Replace,
+				},
+			},
+			output: labels.FromMap(map[string]string{
+				"a": "foo",
+			}),
+		},
 		{
 			input: labels.FromMap(map[string]string{
 				"a": "foo",