public int http2InitialStreamWindowSize() {
        return get(HTTP2_INITIAL_STREAM_WINDOW_SIZE);
    }
Returns the <a href="https://datatracker.ietf.org/doc/html/rfc7540#section-6.5.2">SETTINGS_INITIAL_WINDOW_SIZE</a>
for HTTP/2 stream-level flow control.
core/src/main/java/com/linecorp/armeria/client/ClientFactoryOptions.java