Propagate resolver attributes from cdsbalancer via the addresses.

[anicr7]

To make the propagation easier add a helper method in attributes library which merges two attributes together.

This feature allows any resolver attributes to be propagated further along the xds layers. This is especially useful for resolvers that embed the xds resolvers for example and want to propagated custom attributes.

@dfawley

[codecov[bot]]

Codecov Report

Merging #6945 (87492de) into master (5051eea) will decrease coverage by 0.21%. The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6945      +/-   ##
==========================================
- Coverage   83.81%   83.60%   -0.21%     
==========================================
  Files         287      287              
  Lines       30930    30957      +27     
==========================================
- Hits        25925    25883      -42     
- Misses       3952     4003      +51     
- Partials     1053     1071      +18     

Files	Coverage Δ
attributes/attributes.go	100.00% <100.00%> (ø)
xds/internal/balancer/cdsbalancer/cdsbalancer.go	81.20% <100.00%> (+0.40%)	:arrow_up:

... and 20 files with indirect coverage changes

[dfawley]

I'm pretty sure it's decided that we will not be pursuing this approach anymore.