WIP: PoC Container Stats

[pablochacin]

What does this PR do?

This PR is a proof of concept for gathering resource consumption stats for a container.

This is a WIP. The intention is to show the general approach and highlight some issues that need to be addressed.

Open Issues:

• The new function was added to the Docker provider because adding it to the Container interface would break other implementations of the interface such as the reaperContainer
• Calculating the CPU percentage requires two samples of the stats. Currently, this is implemented by issuing two requests separate by a hardcoded waiting time of 1s.
  One alternative implementation could be passing the previous sample to the stats call. If it is nil, the instantaneous stats will be returned (with the CPU% as 0)

 base := provider.Stats(containerId, nil)     // returns CPU percentage = 0
 // execute test
 stats := provider.Stats(containerId, base)  // returns CPU percentage

• Testing the new feature is challenging. A container's resource consumption varies depending on many factors. Therefore, the statistics retrieved from a container cannot just be compared with an expected value.

• This code has not yet been tested for Windows

Why is it important?

In some use cases, it would be convenient to have access to the container's stats to measure the resource (memory, CPU) consumption to detect regressions or establish a comparison between alternative implementations or configurations.

Related issues

• Closes [Feature]: Expose containers stats #1826

How to test this PR

In its current form, the suggested way to test is by looking at the CPU stats and see if vales are "reasonable" .
Automating tests is an ongoing effort.

Follow-ups

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