public static boolean defaultUseHttp2Preface() {
        return DEFAULT_USE_HTTP2_PREFACE;
    }
Returns the default value of the {@link ClientFactoryBuilder#useHttp2Preface(boolean)} option.
If enabled, the HTTP/2 connection preface is sent immediately for a cleartext HTTP/2 connection,
reducing an extra round trip incurred by the {@code OPTIONS * HTTP/1.1} upgrade request.
If disabled, the {@code OPTIONS * HTTP/1.1} request with {@code "Upgrade: h2c"} header is sent for
a cleartext HTTP/2 connection. Consider disabling this flag if your HTTP servers have issues
handling or rejecting the HTTP/2 connection preface without a upgrade request.

<p>Note that this option is only effective when the {@link SessionProtocol} of the {@link Endpoint} is
{@link SessionProtocol#HTTP}. This option does not affect ciphertext HTTP/2 connections, which use ALPN
for protocol negotiation, or {@link SessionProtocol#H2C}, which will always use HTTP/2 connection
preface. This option is ignored if a user specified the value explicitly via
{@link ClientFactoryBuilder#useHttp2Preface(boolean)}.

<p>This flag is enabled by default. Specify the
{@code -Dcom.linecorp.armeria.defaultUseHttp2Preface=false} JVM option to disable it.
core/src/main/java/com/linecorp/armeria/common/Flags.java