public String toString()
    {
        return new StringBuilder().append('{').append("extend:").append(
                getExtendedMessageFullName()).append(',').append("fields:").append(fields.values())
                .append('}').toString();
    }
Represents an extend block declared in either the {@link Proto} or nested in a {@link Message}.

@author Philippe Laflamme
protostuff-parser/src/main/java/io/protostuff/parser/Extension.java