[CSM] Remove requirement that servers have to be xDS enabled (#36410)

Closes #36410

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36410 from yashykt:CsmEnableForAllServers cfe8d7db0b
PiperOrigin-RevId: 626501498

src/cpp/ext/csm/csm_observability.cc

@@ -48,9 +48,8 @@ namespace {
 std::atomic<bool> g_csm_plugin_enabled(false);
 }
 
-bool CsmServerSelector(const grpc_core::ChannelArgs& args) {
-  return g_csm_plugin_enabled &&
-         args.GetBool(GRPC_ARG_XDS_ENABLED_SERVER).value_or(false);
+bool CsmServerSelector(const grpc_core::ChannelArgs& /*args*/) {
+  return g_csm_plugin_enabled;
 }
 
 bool CsmChannelTargetSelector(absl::string_view target) {

test/cpp/ext/csm/csm_observability_test.cc

@@ -77,15 +77,9 @@ TEST(CsmChannelTargetSelectorTest, CsmObservabilityOutOfScope) {
       "xds://traffic-director-global.xds.googleapis.com/foo"));
 }
 
-TEST(CsmServerSelectorTest, ChannelArgsWithoutXdsServerArg) {
+TEST(CsmServerSelectorTest, ChannelArgs) {
   auto obs = CsmObservabilityBuilder().BuildAndRegister();
-  EXPECT_FALSE(internal::CsmServerSelector(grpc_core::ChannelArgs()));
-}
-
-TEST(CsmServerSelectorTest, ChannelArgsWithXdsServerArg) {
-  auto obs = CsmObservabilityBuilder().BuildAndRegister();
-  EXPECT_TRUE(internal::CsmServerSelector(
-      grpc_core::ChannelArgs().Set(GRPC_ARG_XDS_ENABLED_SERVER, true)));
+  EXPECT_TRUE(internal::CsmServerSelector(grpc_core::ChannelArgs()));
 }
 
 TEST(CsmServerSelectorTest, CsmObservabilityOutOfScope) {