🐛 debug grpc export (#2379) (#2719)

* 🐛 debug grpc export (#2379) 

#2379 Fixed the issue that the GRPC exporter did not establish an RPC link
原文使用的 otlptracegrpc.NewUnstarted创建的是一个未建立rpc连接的导出器，无法正常使用；改为otlptracegrpc.New才妥

* Update agent_test.go

修复单元测试失败

core/trace/agent.go

@@ -60,11 +60,12 @@ func createExporter(c Config) (sdktrace.SpanExporter, error) {
 	case kindZipkin:
 		return zipkin.New(c.Endpoint)
 	case kindGrpc:
-		return otlptracegrpc.NewUnstarted(
+		return otlptracegrpc.New(
+			context.Background(),
 			otlptracegrpc.WithInsecure(),
 			otlptracegrpc.WithEndpoint(c.Endpoint),
 			otlptracegrpc.WithDialOption(grpc.WithBlock()),
-		), nil
+		)
 	default:
 		return nil, fmt.Errorf("unknown exporter: %s", c.Batcher)
 	}