[xds interop] Use new CSM Observability APIs (#35508)

Closes #35508

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35508 from yashykt:XdsInteropNewCsm 09d408c082
PiperOrigin-RevId: 597588033

test/cpp/interop/xds_interop_client.cc

@@ -403,9 +403,10 @@ void EnableCsmObservability() {
   auto meter_provider =
       std::make_shared<opentelemetry::sdk::metrics::MeterProvider>();
   meter_provider->AddMetricReader(std::move(prometheus_exporter));
-  grpc::experimental::CsmObservabilityBuilder observability;
-  observability.SetMeterProvider(std::move(meter_provider));
-  auto status = observability.BuildAndRegister();
+  grpc::experimental::OpenTelemetryPluginBuilder()
+      .AddPluginOption(grpc::experimental::MakeCsmOpenTelemetryPluginOption())
+      .SetMeterProvider(std::move(meter_provider))
+      .BuildAndRegisterGlobal();
 }
 
 void RunServer(const int port, StatsWatchers* stats_watchers,

test/cpp/interop/xds_interop_server.cc

@@ -54,9 +54,10 @@ void EnableCsmObservability() {
   auto meter_provider =
       std::make_shared<opentelemetry::sdk::metrics::MeterProvider>();
   meter_provider->AddMetricReader(std::move(prometheus_exporter));
-  grpc::experimental::CsmObservabilityBuilder observability;
-  observability.SetMeterProvider(std::move(meter_provider));
-  auto status = observability.BuildAndRegister();
+  grpc::experimental::OpenTelemetryPluginBuilder()
+      .AddPluginOption(grpc::experimental::MakeCsmOpenTelemetryPluginOption())
+      .SetMeterProvider(std::move(meter_provider))
+      .BuildAndRegisterGlobal();
 }
 
 int main(int argc, char** argv) {