public static <T, U> Collector<T, ?, Optional<T>> modeBy(Function<? super T, ? extends U> function) {
        return collectingAndThen(modeAllBy(function), Stream::findFirst);
    }
Get a {@link Collector} that calculates the <code>MODE()</code> function.
jOOL/src/main/java/org/jooq/lambda/Agg.java