Function crypto_rest_client::fetch_l2_snapshot

pub fn fetch_l2_snapshot(
    exchange: &str,
    market_type: MarketType,
    symbol: &str,
    retry: Option<u64>
) -> Result<String, Error>

Fetch level2 orderbook snapshot.

retry None means no retry; Some(0) means retry unlimited times; Some(n) means retry n times.