[release-0.8] metrics: fix collecting hugetlb statistics from cgroup v1

[marquiz]

• Hugetlb statistics collector matched both usage and (later added) reservation accounting files (.rsvd.). When both were present, the collector tried to create duplicate metrics entries: usage and reservation values got the same name and labels.
• This patch ignores .rsvd. files when collecting statistics in order to keep reporting exactly what was reported before them.
• Fixes error "cgroup_hugetlb_usage... collected before with the same name and label values" error in cri-resmgr output.
• Fixes issue #859.

(cherry picked from commit b9e381cc6c9a56e6520b66e6a619e0e08468ce92)

Backports #968

[codecov-commenter]

Codecov Report

Merging #979 (0690f20) into release-0.8 (7d464f9) will decrease coverage by 1.73%. The diff coverage is 0.00%.

@@               Coverage Diff               @@
##           release-0.8     #979      +/-   ##
===============================================
- Coverage        33.68%   31.95%   -1.73%     
===============================================
  Files               61       65       +4     
  Lines             9156     9835     +679     
===============================================
+ Hits              3084     3143      +59     
- Misses            5789     6404     +615     
- Partials           283      288       +5     

Impacted Files	Coverage Δ
pkg/cgroups/cgroupstats.go	0.00% <0.00%> (ø)
...ce-manager/policy/builtin/dynamic-pools/metrics.go	0.00% <0.00%> (ø)
...source-manager/policy/builtin/dynamic-pools/dyp.go	9.35% <0.00%> (ø)
...source-manager/policy/builtin/dynamic-pools/cpu.go	0.00% <0.00%> (ø)
...urce-manager/policy/builtin/dynamic-pools/flags.go	46.42% <0.00%> (ø)
pkg/cpuallocator/allocator.go	48.32% <0.00%> (+0.15%)	:arrow_up:

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