JMeter SummariserRunningSample: The member variable startTime is only updated at initialization

### Expected behavior

update the startTime When calling the addSample method

### Actual behavior

At present, the start time is assigned when the object is created. The creation time of the object is too early, which will lead to incorrect rate calculation. I used jmeter extension, which contains some time-consuming logic, which will lead to greater error. I observed the SamplingStatCalculator class. When it calls the addSample method, it updates the start time in real time. Is this the expected behavior?

### Steps to reproduce the problem

For some reason, I cannot submit my extension code to you, but as I describe the actual behavior, this problem is very clear. If this is not a bug, please tell me why it is designed like this. Thank you very much, especially for being able to read my poor English description patiently

### JMeter Version

5.2.1

### Java Version

1.8.0_162

### OS Version

win 7