use AsyncCommands;
use Connection;
/**
* Adds a key to Redis if it does not already exist.
* This function attempts to set a key with a value in Redis using the `SETNX` command,
* which sets the key only if it does not already exist.
*
* # Arguments
* * `conn` - A mutable reference to a Redis connection.
* * `key` - The key to be added.
* * `value` - The value to be associated with the key.
*
* # Returns
* * `Ok(())` if the key was successfully added.
* * `Err(redis::RedisError)` if the key already exists or if there was an error during the operation.
*/
pub async