xds: promote XdsNameResolver2

[voidzcy]

Tests look good.

One thing I noticed is

grpc-java/xds/src/main/java/io/grpc/xds/XdsNameResolver2.java

Line 243 in cb8b067

"Generated service config (method config):\n{0}", new Gson().toJson(rawServiceConfig));

might be too verbose, every single RPC will print out this log. We may want to delete it or lower its level. But that should be done in a separate PR. This PR only replaces XdsNameResolver with XdsNameResolver2. Diff is misleading.

[dapengzhang0]

This PR only replaces XdsNameResolver with XdsNameResolver2. Diff is misleading.

Actually you can only make one-line change by modifying resources/META-INF/services/io.grpc.NameResolverProvider

[voidzcy]

Actually you can only make one-line change by modifying resources/META-INF/services/io.grpc.NameResolverProvider

That would still leave two copies of code, in which the old copy eventually needs to be deleted.

[dapengzhang0]

That would still leave two copies of code, in which the old copy eventually needs to be deleted.

We leave two copies of code until the new resolver is working for at lease one release.

[dapengzhang0]

Plus, one-line change is super easy to rollback, even new change is added to the resolver later.

[voidzcy]

Plus, one-line change is super easy to rollback, even new change is added to the resolver later.

We do not need to wait one release. We should delete the unused codepath as quick as possible. Otherwise, if we need to change something in the resolver, we need to change in both, just like the issue #7400 is facing.

[dapengzhang0]

LGTM