Add KEEP_LABELS option for whitelisting labels

This allows to provide a list of labels that are the only labels allowed in the UI, reversing the logic of STRIP_LABELS.
Fixes #170

internal/transform/strip.go

@@ -9,10 +9,16 @@ import (
 // StripLables allows filtering out some labels from alerts
 // it takes the list of label keys to ignore and alert label map
 // it will return label map without labels found on the ignore list
-func StripLables(ignoredLabels []string, sourceLabels map[string]string) map[string]string {
+func StripLables(keptLabels, ignoredLabels []string, sourceLabels map[string]string) map[string]string {
+	// empty keep list means keep everything by default
+	keepAll := len(keptLabels) == 0
 	labels := map[string]string{}
 	for label, value := range sourceLabels {
-		if !slices.StringInSlice(ignoredLabels, label) {
+		// is explicitly marked to be kept
+		inKeep := slices.StringInSlice(keptLabels, label)
+		// is explicitly marked to be stripped
+		inStrip := slices.StringInSlice(ignoredLabels, label)
+		if (keepAll || inKeep) && !inStrip {
 			// strip leading and trailung space in label value
 			// this is to normalize values in case space is added by Alertmanager rules
 			labels[label] = strings.TrimSpace(value)

internal/transform/strip_test.go

@@ -9,6 +9,7 @@ import (
 
 type stripTest struct {
 	strip  []string
+	keep   []string
 	before map[string]string
 	after  map[string]string
 }
@@ -16,6 +17,7 @@ type stripTest struct {
 var stripTests = []stripTest{
 	stripTest{
 		strip: []string{"env"},
+		keep:  []string{},
 		before: map[string]string{
 			"host":  "localhost",
 			"env":   "production",
@@ -28,6 +30,7 @@ var stripTests = []stripTest{
 	},
 	stripTest{
 		strip: []string{"server"},
+		keep:  []string{},
 		before: map[string]string{
 			"host":  "localhost",
 			"env":   "production",
@@ -41,6 +44,7 @@ var stripTests = []stripTest{
 	},
 	stripTest{
 		strip: []string{},
+		keep:  []string{},
 		before: map[string]string{
 			"host":  "localhost",
 			"env":   "production",
@@ -54,16 +58,50 @@ var stripTests = []stripTest{
 	},
 	stripTest{
 		strip: []string{"host"},
+		keep:  []string{},
 		before: map[string]string{
 			"host": "localhost",
 		},
 		after: map[string]string{},
 	},
+	stripTest{
+		strip: []string{},
+		keep:  []string{"env"},
+		before: map[string]string{
+			"host":  "localhost",
+			"env":   "production",
+			"level": "info",
+		},
+		after: map[string]string{
+			"env": "production",
+		},
+	},
+	stripTest{
+		strip: []string{"env"},
+		keep:  []string{"host"},
+		before: map[string]string{
+			"host":  "localhost",
+			"env":   "production",
+			"level": "info",
+		},
+		after: map[string]string{
+			"host": "localhost",
+		},
+	},
+	stripTest{
+		strip: []string{},
+		keep:  []string{"env"},
+		before: map[string]string{
+			"host":  "localhost",
+			"level": "info",
+		},
+		after: map[string]string{},
+	},
 }
 
 func TestStripLables(t *testing.T) {
 	for _, testCase := range stripTests {
-		labels := transform.StripLables(testCase.strip, testCase.before)
+		labels := transform.StripLables(testCase.keep, testCase.strip, testCase.before)
 		if !reflect.DeepEqual(labels, testCase.after) {
 			t.Errorf("StripLables failed, expected %v, got %v", testCase.after, labels)
 		}