exclude cache value from container memory utilization metric

[emilymcafee]

Summary

Closes #280 by excluding cache usage from memory utilization stat.

Implementation details

Subtract MemoryStats.Stats.Cache from MemoryStats.Usage and use the result as the memory stat.

Testing

I have not been able to get the test suite to run successfully on master/dev or on the fork. I'm new to go, so guessing this is an issue with my local configuration 😅 A few things that I've been able to test successfully:

• The unit test for the affected file.
• Build an image from the fork + override the agent on a running container instance, confirm via Docker pseudo-files & CloudWatch that the actual usage metric is reported instead of the metric that includes the cache.

I'm happy to keep trying to get the suite to run if this is a prerequisite for a PR, but would really appreciate some pointers on how to get tests running in a fresh environment (any gotchas with versioning/dependencies?)

• Builds (make release)
• Unit tests (make short-test) pass
• Integration tests (make test and make run-integ-tests) pass
• Functional tests (make run-functional-tests) pass

New tests cover the changes: Yes

Description for the changelog

Fix memory metric to exclude cache value.

Licensing

This contribution is under the terms of the Apache 2.0 License: Yes

[theotherian]

Thanks for working on this! This issue has been causing our dev organization some headaches; looking forward to seeing it fixed!

[aaithal]

@emilymcafee Thanks a lot for this PR. As there have been no concerns raised in #280 for this, I'm merging this as everything seems to be in order as far as tests are concerned.

[emilymcafee]

\o/ thanks @aaithal !