There is a little mismatch between the variables name here:
void setTimestamp(long int seconds)
{
_timestamp = " " + String(seconds) + "000000000";
}`
String toString() const { return _measurement + _tags + _values + _time; }
private:
String _measurement;
String _tags;
String _values;
String _timestamp;
};
I solved changing from _time to _timestamp