pub fn initial_stream_window_size(self, sz: impl Into<Option<u32>>) -> Self {
        Endpoint {
            init_stream_window_size: sz.into(),
            ..self
        }
    }
Sets the [`SETTINGS_INITIAL_WINDOW_SIZE`][spec] option for HTTP2
stream-level flow control.

Default is 65,535

[spec]: https://httpwg.org/specs/rfc9113.html#InitialWindowSize
tonic/src/transport/channel/endpoint.rs