public boolean useHttp2Preface() {
        return get(USE_HTTP2_PREFACE);
    }
Returns whether to send an HTTP/2 preface string instead of an HTTP/1 upgrade request to negotiate
the protocol version of a cleartext HTTP connection.

<p>Note that this option is only effective when the {@link SessionProtocol} of the {@link Endpoint} is
{@link SessionProtocol#HTTP}.
If the {@link SessionProtocol} is {@link SessionProtocol#HTTPS} or {@link SessionProtocol#H2}, ALPN will
be used. If the {@link SessionProtocol} is {@link SessionProtocol#H2C}, the client will always use
HTTP/2 connection preface.
core/src/main/java/com/linecorp/armeria/client/ClientFactoryOptions.java