relabel: refactor: extract config.Validate method

And add a test for it, which fails because validation is not strong enough.

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

model/relabel/relabel.go

@@ -108,6 +108,10 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if c.Regex.Regexp == nil {
 		c.Regex = MustNewRegexp("")
 	}
+	return c.Validate()
+}
+
+func (c *Config) Validate() error {
 	if c.Action == "" {
 		return fmt.Errorf("relabel action cannot be empty")
 	}

model/relabel/relabel_test.go

@@ -14,6 +14,7 @@
 package relabel
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/prometheus/common/model"
@@ -575,6 +576,77 @@ func TestRelabel(t *testing.T) {
 	}
 }
 
+func TestRelabelValidate(t *testing.T) {
+	tests := []struct {
+		config   Config
+		expected string
+	}{
+		{
+			config:   Config{},
+			expected: `relabel action cannot be empty`,
+		},
+		{
+			config: Config{
+				Action: Replace,
+			},
+			expected: `requires 'target_label' value`,
+		},
+		{
+			config: Config{
+				Action: Lowercase,
+			},
+			expected: `requires 'target_label' value`,
+		},
+		{
+			config: Config{
+				Action:      Lowercase,
+				Replacement: DefaultRelabelConfig.Replacement,
+				TargetLabel: "${3}",
+			},
+			expected: `"${3}" is invalid 'target_label'`,
+		},
+		{
+			config: Config{
+				SourceLabels: model.LabelNames{"a"},
+				Regex:        MustNewRegexp("some-([^-]+)-([^,]+)"),
+				Action:       Replace,
+				Replacement:  "${1}",
+				TargetLabel:  "${3}",
+			},
+		},
+		{
+			config: Config{
+				SourceLabels: model.LabelNames{"a"},
+				Regex:        MustNewRegexp("some-([^-]+)-([^,]+)"),
+				Action:       Replace,
+				Replacement:  "${1}",
+				TargetLabel:  "0${3}",
+			},
+			expected: `"0${3}" is invalid 'target_label'`,
+		},
+		{
+			config: Config{
+				SourceLabels: model.LabelNames{"a"},
+				Regex:        MustNewRegexp("some-([^-]+)-([^,]+)"),
+				Action:       Replace,
+				Replacement:  "${1}",
+				TargetLabel:  "-${3}",
+			},
+			expected: `"-${3}" is invalid 'target_label' for replace action`,
+		},
+	}
+	for i, test := range tests {
+		t.Run(fmt.Sprint(i), func(t *testing.T) {
+			err := test.config.Validate()
+			if test.expected == "" {
+				require.NoError(t, err)
+			} else {
+				require.ErrorContains(t, err, test.expected)
+			}
+		})
+	}
+}
+
 func TestTargetLabelValidity(t *testing.T) {
 	tests := []struct {
 		str   string