fix(common): create default gRPC credentials only if needed (#10280)

For backwards compatibility with the pre-Unified Credentials world we
were calling `grpc::GoogleDefaultCredentials()` if
`g::c::GrpcCredentialsOption` was not set. In many cases the option is
not set, but `g::c::UnifiedCredentialsOption` is. If that is the case,
the unified options are preferred. Calling
`grpc::GoogleDefaultCredentials()` produces spurious errors, and these
credentials won't be used.

google/cloud/internal/populate_grpc_options.cc

@@ -29,7 +29,8 @@ Options PopulateGrpcOptions(Options opts, std::string const& emulator_env_var) {
       opts.set<GrpcCredentialOption>(grpc::InsecureChannelCredentials());
     }
   }
-  if (!opts.has<GrpcCredentialOption>()) {
+  if (!opts.has<GrpcCredentialOption>() &&
+      !opts.has<UnifiedCredentialsOption>()) {
     opts.set<GrpcCredentialOption>(grpc::GoogleDefaultCredentials());
   }
   if (!opts.has<GrpcTracingOptionsOption>()) {