xds/internal/xdsclient: Add counter metrics for valid and invalid resource updates

[zasweq]

This PR adds counter metrics for the xDS Client, and unit tests. E2E tests still need to be written.

The gauge metrics are blocked on adding support for asynchronous gauges, and we also should move our synchronous gauges in RLS to asynchronous.

RELEASE NOTES:

• xds/internal/xdsclient: Add counter metrics for valid and invalid resource updates

[codecov]

Codecov Report

Attention: Patch coverage is 91.48936% with 4 lines in your changes missing coverage. Please review.

Project coverage is 82.19%. Comparing base (2fd426d) to head (23257e2).

Files with missing lines	Patch %	Lines
internal/stats/metrics_recorder_list.go	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8038      +/-   ##
==========================================
+ Coverage   82.17%   82.19%   +0.02%     
==========================================
  Files         383      383              
  Lines       38776    38807      +31     
==========================================
+ Hits        31864    31899      +35     
+ Misses       5585     5580       -5     
- Partials     1327     1328       +1     

Files with missing lines	Coverage Δ
resolver/resolver.go	100.00% <ø> (ø)
resolver_wrapper.go	84.29% <100.00%> (+0.13%)	⬆️
server.go	82.35% <100.00%> (-0.20%)	⬇️
xds/internal/resolver/xds_resolver.go	79.04% <100.00%> (+0.38%)	⬆️
xds/internal/xdsclient/authority.go	77.11% <100.00%> (+0.24%)	⬆️
xds/internal/xdsclient/clientimpl.go	80.81% <100.00%> (+0.48%)	⬆️
xds/internal/xdsclient/pool.go	81.11% <100.00%> (+0.65%)	⬆️
xds/server.go	82.42% <100.00%> (+0.21%)	⬆️
internal/stats/metrics_recorder_list.go	91.48% <20.00%> (-8.52%)	⬇️

... and 17 files with indirect coverage changes