Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.87%. Comparing base (5666049) to head (5ebc9a7).
Report is 21 commits behind head on master.

@@            Coverage Diff             @@
##           master    #7586      +/-   ##
==========================================
+ Coverage   81.74%   81.87%   +0.12%     
==========================================
  Files         361      361              
  Lines       27813    27827      +14     
==========================================
+ Hits        22736    22783      +47     
+ Misses       3868     3848      -20     
+ Partials     1209     1196      -13     

(Sorry, hit send too soon; updated comment above.)

Could you add a test that works at an even higher level? I.e. registers the creds builder from outside of the xdsclient package and confirms that a grpc client created using grpc.NewClient will invoke the proper dialer?

Done.
In latest commit, I added a new e2e test test/xds/xds_client_custom_dialer_test.go that registers a creds builder (creds bundle with a custom Dialer method that simply acts as a pass-through), sets up a xDS management server, creates a new xDS bootstrap that specifies the creds builder name as part of the channel_creds, creates a xDS resolver that uses the bootstrap, creates a test backend, creates a ClientConn via grpc.NewClient, make a successful RPC call, finally verify the custom Dialer has been called. This works end-to-end as the custom Dialer is just a pass-through.

The test setup is inspired by other tests under test/xds/.

+@easwars for a second review