@Nullable
    public ServerPort activePort(SessionProtocol protocol) {
        return activePort0(requireNonNull(protocol, "protocol"));
    }
Returns the primary {@link ServerPort} which serves the given {@link SessionProtocol}
that this {@link Server} is listening to. If this {@link Server} has both a local port and
a non-local port, the non-local port is returned.

@return the primary {@link ServerPort}, or {@code null} if there is no active port available for
        the given {@link SessionProtocol}.
core/src/main/java/com/linecorp/armeria/server/Server.java