[OTel C++] Allow setting only tracing with non-global Build experimental API

Author: yashykt

src/cpp/ext/otel/otel_plugin.cc

@@ -264,9 +264,9 @@ absl::Status OpenTelemetryPluginBuilderImpl::BuildAndRegisterGlobal() {
 
 absl::StatusOr<std::shared_ptr<grpc::experimental::OpenTelemetryPlugin>>
 OpenTelemetryPluginBuilderImpl::Build() {
-  if (meter_provider_ == nullptr) {
+  if (meter_provider_ == nullptr && tracer_provider_ == nullptr) {
     return absl::InvalidArgumentError(
-        "Need to configure a valid meter provider.");
+        "Need to configure a valid meter provider or tracer provider.");
   }
   return std::make_shared<OpenTelemetryPluginImpl>(
       metrics_, meter_provider_, std::move(target_attribute_filter_),