[CONTINT-3554] Fix build on linux and do not send cgroup node inode if in host cgroup namespace

[AliDatadog]

What does this PR do?

This PR adds build flags in the logic that retrieves the container id since it only works on linux.
It also only retrieves the cgroup node inode if the sender not in the host cgroup namespace. Indeed it would mean either that:

• We're not in a container and the inode we're sending is not related to a container
• We already have access to the container-id and it will be retrieved in the previous logic

Motivation

Fix build and do not emit data if not necessary (limit perf impact)

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-12-19 18:40:33

Comparing candidate commit d27c2f9 in PR branch ali/fix-cid-retrieval-on-host with baseline commit 4ae528b in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 38 metrics, 2 unstable metrics.

scenario:BenchmarkSingleSpanRetention/with-rules/match-all-24

• 🟥 execution_time [+5.740µs; +7.442µs] or [+2.302%; +2.984%]

[nsrip-dd]

Thanks! One change needed for the test file build tags (see my comment). Should someone with cgroups knowledge review the other changes?

[nsrip-dd]

The test build tag isn't defined, so these tests won't run. I think you just want the linux tag.

[AliDatadog]

Thanks. I just removed it !

I also asked a review to my team members. They should have the knowledge necessary. Normally I am just aligning on what is done in dogstatsd DataDog/datadog-go#291

[nsrip-dd]

Changes look similar to the linked datadog-go changes, which were approved by someone with more knowledge, so LGTM