long java9CurrentTimeMicros() {
        final Instant now = Clock.systemUTC().instant();
        return TimeUnit.SECONDS.toMicros(now.getEpochSecond()) +
               TimeUnit.NANOSECONDS.toMicros(now.getNano());
    }
Returns a {@link CompletableFuture} which executes all callbacks with the {@link RequestContext}.
core/src/main/java/com/linecorp/armeria/internal/common/JavaVersionSpecific.java