Function init 

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

Initialize the global FRED client with an API key.

Optional if FRED_API_KEY is set: series falls back to that env var on first use, and a later init call still errors as already initialized.

Arguments

• api_key - Your FRED API key (free at https://fred.stlouisfed.org/docs/api/api_key.html)

Errors

Returns FinanceError::InvalidParameter if already initialized.