Function init 

pub fn init(api_key: impl Into<String>) -> Result<()>

Initialize the global Alpha Vantage client with an API key.

Must be called once before using any query functions. Subsequent calls return an error.

Arguments

• api_key - Your Alpha Vantage API key (free at https://www.alphavantage.co/support/#api-key)

Errors

Returns crate::FinanceError::InvalidParameter if already initialized.