Log when duplicate async instruments are registered

[jack-berg]

If I use the API to register two async instruments with the same name, but different callback behavior, the first one registered wins. The callback of the second is never invoked, and there are no logs to tell me that.

For example:

    AtomicLong countUp = new AtomicLong(0);
    AtomicLong countDown = new AtomicLong(100);
    meter.gaugeBuilder("foo").buildWithCallback(observe -> observe.record(countUp.incrementAndGet()));
    meter.gaugeBuilder("foo").buildWithCallback(observe -> observe.record(countDown.decrementAndGet()));

We should provide useful log messages when conflicts like this happen since unlike synchronous instruments, we can't just return the same instrument that was previously registered.