Function esp_http_client_get_content_length 

pub unsafe extern "C" fn esp_http_client_get_content_length(
    client: esp_http_client_handle_t,
) -> i64

@brief Get http response content length (from header Content-Length) the valid value if this function invoke after esp_http_client_perform

@param[in] client The esp_http_client handle

@return - (-1) Chunked transfer - Content-Length value as bytes