In #32294 , we accidentally stopped
plumbing `grpc.testing.google_c2p_resolver_metadata_server_override` to
the metadata server http queries. So, we stopped actually using the fake
metadata servers injected in
https://github.com/grpc/grpc/blob/b3d68001f7420ccb47615ed88ac79b4c0456f897/test/core/client_channel/resolvers/google_c2p_resolver_test.cc#L48.

The brings that back.

This code is not plumbed through yet, but it provides the core
infrastructure needed to detect the proper GCP environment resources
needed to set up the labels/attributes/resources for stats, tracing and
logging.

Details on how the various environment resources are setup has been
derived by looking at java's cloud logging library and OpenTelemetry's
future plans. (Could be better explained in an offline review since some
links are internal).

Requesting @veblush for a full review and @markdroth for a structural
review.

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In grpc#32294 , we accidentally stopped
plumbing `grpc.testing.google_c2p_resolver_metadata_server_override` to
the metadata server http queries. So, we stopped actually using the fake
metadata servers injected in
https://github.com/grpc/grpc/blob/b3d68001f7420ccb47615ed88ac79b4c0456f897/test/core/client_channel/resolvers/google_c2p_resolver_test.cc#L48.

The brings that back.

This code is not plumbed through yet, but it provides the core
infrastructure needed to detect the proper GCP environment resources
needed to set up the labels/attributes/resources for stats, tracing and
logging.

Details on how the various environment resources are setup has been
derived by looking at java's cloud logging library and OpenTelemetry's
future plans. (Could be better explained in an offline review since some
links are internal).

Requesting @veblush for a full review and @markdroth for a structural
review.

<!--

If you know who should review your pull request, please assign it to
that
person, otherwise the pull request would get assigned randomly.

If your pull request is for a specific language, please add the
appropriate
lang label.

-->

In #32294 , we accidentally stopped
plumbing `grpc.testing.google_c2p_resolver_metadata_server_override` to
the metadata server http queries. So, we stopped actually using the fake
metadata servers injected in
https://github.com/grpc/grpc/blob/b3d68001f7420ccb47615ed88ac79b4c0456f897/test/core/client_channel/resolvers/google_c2p_resolver_test.cc#L48.

The brings that back.