public static ClientAddressSource ofProxyProtocol() {
        return PROXY_PROTOCOL;
    }
Returns the {@link ClientAddressSource} which indicates the source address specified in
a <a href="https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt">PROXY protocol</a> message
will be used to determine a client address.
core/src/main/java/com/linecorp/armeria/server/ClientAddressSource.java