Add new thread variable __jmv_THREAD_START_TIME_ITERATION contains the currentTimeMillis when the thread iteration start.

[vdaburon]

Corresponding to the issue (new feature): #6319

Description

Add a new unmodifiable JMeter Thread variable named "__jmv_THREAD_START_TIME_ITERATION"

Motivation and Context

It is interesting to know the start timestamp of a thread iteration to calculate cadences or pacing.

This variable can also be used to help other Thread Groups calculate cadences (E.g : Concurrency Thread Group or may be Open Model Thread Group)

How Has This Been Tested?

Yes, add a groovy sampler first sampler of the thread group and compute the delta between the value in the variable __jmv_THREAD_START_TIME_ITERATION and the current time System.currentTimeMillis() in the groovy sampler.

Groovy code
+++++++++++++++++++++++++++
long currentTime = System.currentTimeMillis();
long startThreadIter = Long.parseLong(vars.get("__jmv_THREAD_START_TIME_ITERATION"));
long delta = currentTime - startThreadIter;
return delta;
+++++++++++++++++++++++++++
Results 0 or 1 ms

Screenshots (if appropriate):

Types of changes

• New feature

Checklist:

• [x ] My code follows the code style of this project.
• [ x] I have updated the documentation accordingly.
  Add new variable __jmv_THREAD_START_TIME_ITERATION in the documentation :
  functions.xml, section 6 Pre-defined Variables

[FSchumacher]

Is this to fix some older code?

[vdaburon]

Is this to fix some older code?
Yes, when i search this variable "__jmv_SAME_USER" in the documentation, i don't see it.
So i add this line about the variable __jmv_SAME_USER.
And i add the new one variable "__jmv_THREAD_START_TIME_ITERATION"

[FSchumacher]

I don't think, that this can be used by OMTG, as the thread is started by OMTG itself and not initialised, when that information is needed. But that problem is hopefully addressed by #6353 .

Apart from that, the information might be helpful for listeners or users, so LGTM.