default void connectFailed(SessionProtocol protocol, Endpoint endpoint,
                               SocketAddress sa, Throwable throwable) {}
Called to indicate a connection attempt to the specified {@link SessionProtocol}
and {@link Endpoint} has failed.

@param protocol the protocol associated with the endpoint
@param endpoint an endpoint containing the requested host and port
@param sa the remote socket address of the proxy server
@param throwable the cause of the failure
core/src/main/java/com/linecorp/armeria/client/proxy/ProxyConfigSelector.java