Add span attribute request_body.

fcm.go

@@ -225,6 +225,10 @@ func (c *Client) doSendRequest(ctx context.Context, req SendRequest, loggerEnabl
 	)
 
 	if response.StatusCode != http.StatusOK {
+		span.SetAttributes(
+			attribute.String("request_body", string(data)),
+		)
+
 		err := errors.Errorf("Status is not OK. Status: %d. Body: %q.", response.StatusCode, bodyStr)
 		span.SetStatus(codes.Error, err.Error())
 		span.RecordError(err)