xds: add LRS named metrics support

[danielzhaotongliu]

Design:

• go/grpc-xds-lrs-named-metrics
• go/lrs-named-metrics-design-review-slides
• Implements gRFC A64 https://github.com/grpc/proposal/blob/master/A64-lrs-custom-metrics.md
• Only include named metrics from OrcaLoadReport and remove all previous fields (e.g. CPU utilization, memory utilization, etc.) as they are unused
• Added xDS clusterImpl balancer tests to verify named metrics aggregation logic
• Added completely missing tests for UpstreamLocalityStats to verify aggregation logic

Tested:

• VET_SKIP_PROTO=1 ./vet.sh
• go test -cpu 1,4 -timeout 7m ./...
• go test -race -cpu 1,4 -timeout 7m ./...
• Tested end-to-end by spinning up test client/server and verified named metrics from the OrcaLoadReport are aggregated and send in LRS load reports

RELEASE NOTES:

• xds: implement LRS named metrics support (gRFC A64)

[codecov]

Codecov Report

Merging #7027 (a71207c) into master (f7c5e6a) will decrease coverage by 0.05%.
Report is 5 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7027      +/-   ##
==========================================
- Coverage   82.48%   82.43%   -0.05%     
==========================================
  Files         296      296              
  Lines       31468    31463       -5     
==========================================
- Hits        25955    25937      -18     
- Misses       4457     4460       +3     
- Partials     1056     1066      +10     

Files	Coverage Δ
xds/internal/balancer/clusterimpl/picker.go	96.73% <100.00%> (+8.07%)	⬆️

... and 22 files with indirect coverage changes

[dfawley]

Thanks for the PR!