Quick comment first:

all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names

so technically, _GRPC_XDS_USER_AGENT_VERSION_SUFFIX is reserved.

In general, for Bazel and cmake, you'll want to add defines in the corresponding library target. Since the change you're doing does not need to propagate to the users of the library, the change is luckily less problematic than if it was to pollute the whole environment.

For bazel you want to use the local_defines attribute for the build target.

For cmake, that'd be add_definitions used like so. I don't think we could make it easy to only target the one library, but at the same time, I don't think it matters that much here.

It looks like there are going to be complications in setting this macro properly for C++, because bazel is used for both C++ and Python, and cmake is used for both C++ and C#, so in both cases, we need some way to set the macro conditionally.

@jtattermusch says that we can handle cmake by passing the defines to cmake from the scripts that are used to build C#. He said he'd send me a separate PR for that later this week.

Bazel will require more thought, because there's no obvious way to set the macros conditionally for a target that is common to both python and C++. So I think I'm going to try to handle that in a separate PR as well.

Let's move forward with this PR as-is, just providing the macros but not actually setting them. That way, the wrapped langs can start using this while we continue to figure out how to set it properly in bazel and cmake.

As per today's discussion, we'll leave these macros unset for C++, setting them only for wrapped languages when we know that we're building for those languages. This means that we'll be lumping in cases where we build python with bazel in with the C++ stats, but that's probably okay for now. We can always try to find a new way to optimize that later if we need more detail.

Known issues: #26019