private static boolean isSampled() {
        return Flags.verboseExceptionSampler().isSampled(ClosedSessionException.class);
    }
Returns a {@link ClosedSessionException} which may be a singleton or a new instance, depending on
{@link Flags#verboseExceptionSampler()}'s decision.
core/src/main/java/com/linecorp/armeria/common/ClosedSessionException.java