@Nullable
    public Throwable unavailabilityCause() {
        return provider.unavailabilityCause();
    }
Returns why this {@link TransportType} is not available.

@return why this {@link TransportType} is not available, or {@code null} if this {@link TransportType}
        is currently available.

@see #isAvailable()
core/src/main/java/com/linecorp/armeria/common/util/TransportType.java