private static String appendReason(String message, @Nullable String reason) {
        if (reason == null) {
            return message;
        }
        return message + ", reason: " + reason;
    }
Returns the actual {@link SessionProtocol}.

@return the actual {@link SessionProtocol}, or {@code null} if failed to determine the protocol.
core/src/main/java/com/linecorp/armeria/client/SessionProtocolNegotiationException.java