Expose single annotation/label via downward API

pkg/fieldpath/fieldpath.go

@@ -42,7 +42,20 @@ func ExtractFieldPathAsString(obj interface{}, fieldPath string) (string, error)
 		return "", nil
 	}
 
-	switch fieldPath {
+	path, subscript := SplitMaybeSubscriptedPath(fieldPath)
+
+	if len(subscript) > 0 {
+		switch path {
+		case "metadata.annotations":
+			return accessor.GetAnnotations()[subscript], nil
+		case "metadata.labels":
+			return accessor.GetLabels()[subscript], nil
+		default:
+			return "", fmt.Errorf("fieldPath %q does not support subscript", fieldPath)
+		}
+	}
+
+	switch path {
 	case "metadata.annotations":
 		return FormatMap(accessor.GetAnnotations()), nil
 	case "metadata.labels":
@@ -57,3 +70,29 @@ func ExtractFieldPathAsString(obj interface{}, fieldPath string) (string, error)
 
 	return "", fmt.Errorf("unsupported fieldPath: %v", fieldPath)
 }
+
+// SplitMaybeSubscriptedPath checks whether the specified fieldPath is
+// subscripted, and
+//  - if yes, this function splits the fieldPath into path and subscript, and
+//    returns (path, subscript).
+//  - if no, this function returns (fieldPath, "").
+//
+// Example inputs and outputs:
+//  - "metadata.annotations['myKey']" --> ("metadata.annotations", "myKey")
+//  - "metadata.annotations['a[b]c']" --> ("metadata.annotations", "a[b]c")
+//  - "metadata.labels"               --> ("metadata.labels", "")
+//  - "metadata.labels['']"           --> ("metadata.labels", "")
+func SplitMaybeSubscriptedPath(fieldPath string) (string, string) {
+	if !strings.HasSuffix(fieldPath, "']") {
+		return fieldPath, ""
+	}
+	s := strings.TrimSuffix(fieldPath, "']")
+	parts := strings.SplitN(s, "['", 2)
+	if len(parts) < 2 {
+		return fieldPath, ""
+	}
+	if len(parts[0]) == 0 || len(parts[1]) == 0 {
+		return fieldPath, ""
+	}
+	return parts[0], parts[1]
+}