public static final TracerFactory instance() {
        if (INSTANCE == null) {
            throw new IllegalStateException(String.format("%s not initialized", TracerFactory.class.getSimpleName()));
        }
        return INSTANCE;
    }
Returns the singleton TracerFactory

@return a <code>TracerFactory</code> value
zuul-core/src/main/java/com/netflix/zuul/monitoring/TracerFactory.java