stats: check pointer before using metrics

[fuweid]

Signed-off-by: Wei Fu fuweid89@gmail.com

Ⅰ. Describe what this PR did

Some machine doesn't have cgroup controller, like pids. Therefore,
before we use it from metrics, we need to check the value.

Ⅱ. Does this pull request fix one issue?

none

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

using existing test case.

Ⅳ. Describe how to verify it

wait for CI.

NOTE: the testing needs to disable cgroup function by umount /sys/fs/cgroup/pids something like that. It is kind of hacking. I did it in offline.

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Merging #2878 into master will decrease coverage by 0.04%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master    #2878      +/-   ##
==========================================
- Coverage   69.11%   69.07%   -0.05%     
==========================================
  Files         286      286              
  Lines       17903    17902       -1     
==========================================
- Hits        12374    12365       -9     
+ Misses       4124     4123       -1     
- Partials     1405     1414       +9

Flag	Coverage Δ
#criv1alpha2_test	39.05% <0%> (-0.01%)	⬇️
#integration_test_0	36.7% <0%> (ø)	⬆️
#integration_test_1	35.37% <0%> (ø)	⬆️
#integration_test_2	36.69% <96.15%> (+0.02%)	⬆️
#integration_test_3	35.48% <96.15%> (+0.06%)	⬆️
#node_e2e_test	34.66% <0%> (-0.16%)	⬇️
#unittest	28.56% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
daemon/mgr/container_stats.go	71.02% <96.15%> (+3.22%)	⬆️
ctrd/watch.go	72.97% <0%> (-5.41%)	⬇️
daemon/logger/crilog/log.go	79.71% <0%> (-2.9%)	⬇️
daemon/containerio/io.go	72.81% <0%> (-1.95%)	⬇️
pkg/meta/store.go	67.44% <0%> (-1.56%)	⬇️
daemon/mgr/container_utils.go	76.76% <0%> (-1.02%)	⬇️
cri/v1alpha2/cri.go	68.81% <0%> (-0.51%)	⬇️
ctrd/container.go	53.91% <0%> (-0.39%)	⬇️
ctrd/supervisord/daemon.go	50.67% <0%> (+1.35%)	⬆️
cri/ocicni/cni_manager.go	61.11% <0%> (+1.85%)	⬆️

[zhuangqh]

LGTM, we must do nil checking before using pointer.