Emit the end-of-iteration metrics outside of the networking and JS code

[na--]

After #1203, the iterations metric is emitted as a part of the netext.NetTrail struct, together with the iteration_duration, data_received, and data_sent metrics. This somewhat makes sense, however it doesn't make sense to call that metric sample container netext.NetTrail (since only 50% of its metrics are network-related). It also doesn't make sense to emit these metrics from the JS code - since they relate to iterations, they should be emitted by the thing that controls iterations...

So, after #1007 is merged, we should refactor the code, probably like this:

• call the SampleContainer something else, IterationMetrics or something like that
• the netext.Dialer would still be needed, but it will only be concerned with the data_sent and data_received metrics, which would then be re-packaged in that new struct
• that struct should be assembled and emitted somewhere outside of the JS code, probably here (that would also allow us to have more accurate non-JS unit tests, for example this one)

[Shehab7osny]

Hi, I would like to work on this issue.
May you please assign it to me?
Thanks!

[na--]

@Shehab7osny, I've assigned you, let us know if you have any questions or problems with this.

[Shehab7osny]

@na-- Thanks, I will start working on this issue.
May I use slack for any clarification or questions?

[na--]

Sure, though you can also just ask any questions here as well 🤷‍♂️