xds/internal/client: Add async gauge metrics for Connected and Resources (A78)

[mbissa]

This PR leverages the async gauge framework and implements the xdsclient metrics to report number of xds resources and whether or not the xDS client currently has a working ADS stream to the xDS server along with the required labels as part of A78

RELEASE NOTES:

• xds/internal/client: Add async gauge metrics for "grpc.xds_client.connected" and "grpc.xds_client.resources" as part of A78.

[codecov]

Codecov Report

❌ Patch coverage is 74.22680% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.23%. Comparing base (c6d5e5e) to head (ec83a4e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/xdsclient/clientimpl.go	25.92%	19 Missing and 1 partial ⚠️
internal/xds/clients/xdsclient/xdsclient.go	92.42%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8807      +/-   ##
==========================================
- Coverage   83.26%   83.23%   -0.03%     
==========================================
  Files         418      418              
  Lines       33004    33101      +97     
==========================================
+ Hits        27480    27551      +71     
- Misses       4109     4135      +26     
  Partials     1415     1415              

Files with missing lines	Coverage Δ
internal/xds/clients/xdsclient/ads_stream.go	85.06% <100.00%> (+0.21%)	⬆️
internal/xds/clients/xdsclient/xdsclient.go	86.61% <92.42%> (+2.03%)	⬆️
internal/xds/xdsclient/clientimpl.go	73.72% <25.92%> (-11.74%)	⬇️

... and 29 files with indirect coverage changes

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