orca: use atomic pointer instead of mutex in server metrics recorder to improve performance

[danielzhaotongliu]

PR included:

• Use atomic pointers instead of mutex to improve performance.
• When the server metrics recorder is modified/written (e.g. SetCPUUtilization) create a new snapshot and atomically store it. When the server metrics recorder is read, return the reference to the latest snapshot. Similar in implementation to C++.

Motivation:

• See internal email thread and benchmarks regarding performance

Tested:

• go test -cpu 1,4 -timeout 7m ./...
• go test -race -cpu 1,4 -timeout 7m ./...
• Verified new implementation improved internal benchmark performance

RELEASE NOTES:

• orca: use atomic pointer to improve performance in server metrics recorder

[codecov]

Codecov Report

Merging #6799 (ba06b28) into master (8645f95) will decrease coverage by 0.12%.
Report is 19 commits behind head on master.
The diff coverage is 87.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6799      +/-   ##
==========================================
- Coverage   83.57%   83.45%   -0.12%     
==========================================
  Files         285      285              
  Lines       30950    30879      -71     
==========================================
- Hits        25865    25771      -94     
- Misses       4021     4036      +15     
- Partials     1064     1072       +8     

Files	Coverage Δ
orca/server_metrics.go	80.92% <87.50%> (ø)

... and 32 files with indirect coverage changes

[danielzhaotongliu]

FYI: @yousukseung