static ContainerTypeSignature ofOptional(TypeSignature elementTypeSignature) {
        requireNonNull(elementTypeSignature, "elementTypeSignature");
        return new ContainerTypeSignature(TypeSignatureType.OPTIONAL, "optional",
                                          ImmutableList.of(elementTypeSignature));
    }
Creates a new type signature for the optional type with the specified element type signature.
core/src/main/java/com/linecorp/armeria/server/docs/TypeSignature.java