Namespace MDC Fields with either labels or tags.

I noticed when I was working with the ECS Java loggers that MDC and/or Metadata fields weren't name spaced.

I can see one of three scenario's:

1. Make the default namespace be `labels` per [https://www.elastic.co/guide/en/ecs/current/ecs-base.html](Base Fields)
2. Make the default namespace be `tags` per [https://www.elastic.co/guide/en/ecs/current/ecs-base.html](Base Fields)
3. Allow for a customized namespace via a configuration parameter.

Here's an example configuration:

```xml

my-application
my-application-cluster-node

environment
dev

```

and a resulting output:

```json
{
"@timestamp": "2021-08-31T15:57:11.355Z",
"log.level": "DEBUG",
"message": "Example with the default Meta Data fields.",
"ecs.version": "1.2.0",
"service.name": "my-application",
"service.node.name": "my-application-cluster-node",
"event.dataset": "my-application.log",
"process.thread.name": "main",
"log.logger": "co.elastic.DemoApplication",
"environment": "dev"
}
```

从 logback EcsEncoder 配置开始，检查 MDC 和 Metadata 字段是如何输出的。将当前 JSON 结构与 ECS 的 labels 和 tags 替代方案进行比较，然后确定实现应使用默认 namespace，还是公开一个可配置的 namespace。namespace 行为达成一致且示例输出反映了该行为后，即表示完成。

由索引模型根据 Issue 内容生成。