Retrieval: Add relabel action to map label names with a regex.

The intended use case is where a user has tags/labels coming
from metadata in Kubernetes or EC2, and wants to make
some subset of them into target labels.

config/config.go

@@ -610,6 +610,8 @@ const (
 	RelabelDrop RelabelAction = "drop"
 	// Sets a label to the modulus of a hash of labels.
 	RelabelHashMod RelabelAction = "hashmod"
+	// Copy labels to other labelnames based on a regex.
+	RelabelLabelMap RelabelAction = "labelmap"
 )
 
 // UnmarshalYAML implements the yaml.Unmarshaler interface.

retrieval/relabel.go

@@ -62,6 +62,19 @@ func relabel(labels clientmodel.LabelSet, cfg *config.RelabelConfig) (clientmode
 		hasher.Write([]byte(val))
 		mod := hasher.Sum64() % cfg.Modulus
 		labels[cfg.TargetLabel] = clientmodel.LabelValue(fmt.Sprintf("%d", mod))
+	case config.RelabelLabelMap:
+		out := make(clientmodel.LabelSet, len(labels))
+		// Take a copy to avoid infinite loops.
+		for ln, lv := range labels {
+			out[ln] = lv
+		}
+		for ln, lv := range labels {
+			if cfg.Regex.MatchString(string(ln)) {
+				res := cfg.Regex.ReplaceAllString(string(ln), cfg.Replacement)
+				out[clientmodel.LabelName(res)] = lv
+			}
+		}
+		labels = out
 	default:
 		panic(fmt.Errorf("retrieval.relabel: unknown relabel action type %q", cfg.Action))
 	}

retrieval/relabel_test.go

@@ -173,6 +173,50 @@ func TestRelabel(t *testing.T) {
 				"d": "58",
 			},
 		},
+		{
+			input: clientmodel.LabelSet{
+				"a":  "foo",
+				"b1": "bar",
+				"b2": "baz",
+			},
+			relabel: []*config.RelabelConfig{
+				{
+					Regex:       &config.Regexp{*regexp.MustCompile("^(b.*)")},
+					Replacement: "bar_${1}",
+					Action:      config.RelabelLabelMap,
+				},
+			},
+			output: clientmodel.LabelSet{
+				"a":      "foo",
+				"b1":     "bar",
+				"b2":     "baz",
+				"bar_b1": "bar",
+				"bar_b2": "baz",
+			},
+		},
+		{
+			input: clientmodel.LabelSet{
+				"a":             "foo",
+				"__meta_my_bar": "aaa",
+				"__meta_my_baz": "bbb",
+				"__meta_other":  "ccc",
+			},
+			relabel: []*config.RelabelConfig{
+				{
+					Regex:       &config.Regexp{*regexp.MustCompile("^__meta_(my.*)")},
+					Replacement: "${1}",
+					Action:      config.RelabelLabelMap,
+				},
+			},
+			output: clientmodel.LabelSet{
+				"a":             "foo",
+				"__meta_my_bar": "aaa",
+				"__meta_my_baz": "bbb",
+				"__meta_other":  "ccc",
+				"my_bar":        "aaa",
+				"my_baz":        "bbb",
+			},
+		},
 	}
 
 	for i, test := range tests {