Function encode_subscribe 

fn encode_subscribe<S, B>(
    session: &S,
    out: &mut PacketWriter<B>,
    packet_id: PacketId,
    subscription_id: Option<u32>,
    max_packet_size: Option<u32>,
) -> Encoded
where
    S: SessionState,
    B: ByteBuffer,

Encodes one pending SUBSCRIBE group straight from the session’s iterator in two passes (length, then bytes), needing no intermediate buffer.