onRequest method
Called when the request is about to be sent.
Implementation
@override
void onRequest(RequestOptions options, RequestInterceptorHandler handler) {
final data = options.data;
final call = _target.startCall(
client: 'dio',
method: options.method,
uri: options.uri,
headers: _flatten(options.headers),
body: data is FormData ? null : _bodyOf(data),
formData: data is FormData ? _formFields(data) : null,
);
if (call != null) options.extra[_callKey] = call;
handler.next(options);
}