@Override public synchronized void onError(Throwable throwable) {
      if (log.isLoggable(Level.INFO)) {
        log.log(Level.INFO, "error from " + call, throwable);
      }
      synchronized (callback) {
        firstError.compareAndSet(null, throwable);
        if (remaining.decrementAndGet() > 0) return;
        callback.onError(firstError.get());
      }
    }
Customizes the aggregated result. For example, summarizing or making immutable.
zipkin/src/main/java/zipkin2/internal/AggregateCall.java