Implement len() Function in ExponentialStatistics for Consistency

[pavelstovicek]

Hi. I'm currently utilizing the len() function in the Statistics module to hold off on presenting outcomes until a minimum of 5 samples are recorded. I wish to replicate this functionality within the ExponentialStatistics module, but I noticed it hasn't been implemented yet. While I realize that self._count might not be necessary for the internal operations of ExponentialStatistics, I believe it would be beneficial to maintain interface consistency across both modules.