Improve collection of Docker memory metrics in Windows

[chrisvanderpennen]

Describe the enhancement:

• Improve module/docker/memory/helper.go to report Windows container memory stats, if present.
• Test if myRawStat.Stats.MemoryStats.Limit is non-zero and only then assign to TotalRssP and UsageP, instead of dividing by zero and assigning NaN. This should avoid breaking JSON outputs.

Describe a specific use case for the enhancement or feature:
Monitoring Windows container memory usage

[exekias]

pinging @narph, I think you wanted to this module a pass on Windows

[chrisvanderpennen]

Looks like this has been affected by #11676 - Windows memory stats don't include limit, so that change will prevent memory stat collection altogether on Windows.

[exekias]

good catch! thank you for sharing. @fearful-symmetry we may need to rethink your fix?

[fearful-symmetry]

@exekias Yah. I tried to find a more elegant way of fixing the NaN issues on Linux, but settled on checking the limit, as the moby source stated that it would never be 0 for a running container. The core problem is that it's possible to get a status object back before a container has started, hence a bunch of junk values and the NaN. Ideally we would find a better way of detecting the invalid status objects such that we don't interfere with valid windows events.

[fearful-symmetry]

Still chewing over how to do this elegantly. After poking around the docker API for a bit, I figure there's two other checks we can get from the stats object that should be platform independent:

OnlineCPUs : count of online CPUs. Is zero during an invalid event.
CPUusage.TotalUsage : Used to calculate deltas for CPU usage percentage. Also zero for an invalid event.

Does any of this sound reasonable @chrisvanderpennen ?

[chrisvanderpennen]

OnlineCPUs is Linux only, sadly. 0 CPUusage.TotalUsage seems a reasonable guard for an invalid stats sample though, I don't see a way for a container in normal operation to have used <1ns of CPU time and my testing on Windows shows it resets to 0 after a running container is stopped.

In case it's useful, here's a gist with json from the stats API on Windows for a started and stopped container.

[fearful-symmetry]

@chrisvanderpennen Thanks a ton! I'm gonna put in a PR for this, so we can at least get this part of the issue squared away.

[fearful-symmetry]

That PR has been merged, so we should be good to move on to adding the actual windows memory stats.

[fearful-symmetry]

That's been merged! I'm gonna close this for now, since it seems like we've squared away the problems in the original issue.