[CONTINT-4562] Container generator

[L3n41c]

What does this PR do?

A form of load can be a container that exists and exposes some data.

This generator aims to provide a way to start a generic container based on user's config.

Motivation

Test the agent performances when it monitors many containers

Related issues

Additional Notes

Follow-up of #1154.

[blt]

I really like where this is headed.

[blt]

Some style comments on the commits. I would like to clarify what happens if the spawned container exits. As I read it the Container generator will not check for spawned container liveness and so if a spawned container fails the user won't know until stop_and_remove_container is called? There's potentially a long period of time between startup and shutdown so an earlier warning -- of outright failure of lading -- is desirable. I could buy may-fail or must-not-fail approaches to this generator, but documenting which approach is taken here is helpful to the user.

[L3n41c]

@blt Thank you for your review!
I should have addressed all the comments.

I would like to clarify what happens if the spawned container exits. As I read it the Container generator will not check for spawned container liveness and so if a spawned container fails the user won't know until stop_and_remove_container is called? There's potentially a long period of time between startup and shutdown so an earlier warning -- of outright failure of lading -- is desirable.

I’ve also updated the PR so that lading is now periodically checking if the spawned containers are still running. If they’re not, a warning is issued.

[blt]

One small comment about using interval instead of repeatedly creating sleeps, otherwise looks good. I'll be happy to see this merge up after that's fixed.