fix(host_metrics source): Include cgroups2 root memory metrics

[bwerthmann]

• cgroup_memory_anon_bytes and cgroup_memory_file_bytes are no longer zero on cgroups v2 systems
• test sources::host_metrics::cgroups::tests::generates_cgroups_metrics should no longer flap

I'm not sure why the is_root check was there for cgroups v2. Memory is only in the root cgroup dir. No child dirs exist in cgroup root on my machine.

Fixes #18697

[bits-bot]

All committers have signed the CLA.

[jszwedko]

Thanks for this @bwerthmann ! We'll review soon.

[bruceg]

Thanks for this fix @bwerthmann. Your explanation makes sense to me. I wrote this initially, and checked the logs as well, and have no evidence why the root check was in there.

[github-actions]

Regression Detector Results

Run ID: 32c0e992-b4c4-4ef7-afaa-d19a8ef36269
Baseline: 69f985d
Comparison: 0efb520
Total CPUs: 7

Performance changes are noted in the perf column of each table:

• ✅ = significantly better comparison variant performance
• ❌ = significantly worse comparison variant performance
• ➖ = no significant change in performance

Significant changes in experiment optimization goals

Confidence level: 90.00%
Effect size tolerance: |Δ mean %| ≥ 5.00%

perf	experiment	goal	Δ mean %	Δ mean % CI
✅	syslog_regex_logs2metric_ddmetrics	ingress throughput	+8.32	[+8.14, +8.50]

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI
✅	syslog_regex_logs2metric_ddmetrics	ingress throughput	+8.32	[+8.14, +8.50]
➖	otlp_http_to_blackhole	ingress throughput	+2.95	[+2.82, +3.08]
➖	socket_to_socket_blackhole	ingress throughput	+2.84	[+2.75, +2.92]
➖	syslog_log2metric_tag_cardinality_limit_blackhole	ingress throughput	+2.46	[+2.34, +2.58]
➖	syslog_humio_logs	ingress throughput	+1.91	[+1.72, +2.11]
➖	http_to_http_acks	ingress throughput	+1.67	[+0.31, +3.04]
➖	http_text_to_http_json	ingress throughput	+1.10	[+0.95, +1.24]
➖	datadog_agent_remap_blackhole_acks	ingress throughput	+1.09	[+0.95, +1.23]
➖	syslog_log2metric_splunk_hec_metrics	ingress throughput	+0.80	[+0.65, +0.96]
➖	datadog_agent_remap_datadog_logs_acks	ingress throughput	+0.77	[+0.69, +0.85]
➖	splunk_hec_route_s3	ingress throughput	+0.53	[+0.04, +1.01]
➖	datadog_agent_remap_datadog_logs	ingress throughput	+0.48	[+0.35, +0.60]
➖	syslog_loki	ingress throughput	+0.20	[+0.10, +0.29]
➖	http_to_http_noack	ingress throughput	+0.17	[+0.07, +0.26]
➖	http_elasticsearch	ingress throughput	+0.07	[-0.01, +0.16]
➖	http_to_http_json	ingress throughput	+0.04	[-0.04, +0.11]
➖	splunk_hec_indexer_ack_blackhole	ingress throughput	+0.00	[-0.14, +0.15]
➖	splunk_hec_to_splunk_hec_logs_acks	ingress throughput	+0.00	[-0.14, +0.14]
➖	splunk_hec_to_splunk_hec_logs_noack	ingress throughput	-0.04	[-0.15, +0.08]
➖	enterprise_http_to_http	ingress throughput	-0.12	[-0.20, -0.05]
➖	datadog_agent_remap_blackhole	ingress throughput	-0.16	[-0.26, -0.05]
➖	http_to_s3	ingress throughput	-0.43	[-0.71, -0.15]
➖	file_to_blackhole	egress throughput	-0.57	[-3.02, +1.88]
➖	syslog_log2metric_humio_metrics	ingress throughput	-0.60	[-0.71, -0.49]
➖	syslog_splunk_hec_logs	ingress throughput	-0.60	[-0.72, -0.48]
➖	otlp_grpc_to_blackhole	ingress throughput	-0.77	[-0.86, -0.69]
➖	fluent_elasticsearch	ingress throughput	-0.86	[-1.35, -0.38]

Explanation

A regression test is an A/B test of target performance in a repeatable rig, where "performance" is measured as "comparison variant minus baseline variant" for an optimization goal (e.g., ingress throughput). Due to intrinsic variability in measuring that goal, we can only estimate its mean value for each experiment; we report uncertainty in that value as a 90.00% confidence interval denoted "Δ mean % CI".

For each experiment, we decide whether a change in performance is a "regression" -- a change worth investigating further -- if all of the following criteria are true:

1. Its estimated |Δ mean %| ≥ 5.00%, indicating the change is big enough to merit a closer look.

2. Its 90.00% confidence interval "Δ mean % CI" does not contain zero, indicating that if our statistical model is accurate, there is at least a 90.00% chance there is a difference in performance between baseline and comparison variants.

3. Its configuration does not mark it "erratic".