[FEATURE]: 希望 Loki Flusher 支持结构化元数据

[callmerichman]

背景
在不索引高基数标签的情况下，在查询时可以使用高基数标签

示例
POST /loki/api/v1/push

{
  "streams": [
    {
      "stream": {
        "label": "value"
      },
      "values": [
          [ "<unix epoch in nanoseconds>", "<log line>", {"trace_id": "0242ac120002", "user_id": "superUser123"}]
      ]
    }
  ]
}

参考
https://grafana.com/docs/loki/latest/get-started/labels/structured-metadata/
https://grafana.com/docs/loki/latest/reference/loki-http-api/#ingest-logs