[EXPORTERS]: elastic search put attributes in json root instead of under

`attributes`

This allows user to set other fields that are part of the [ECS log documentation](https://www.elastic.co/guide/en/ecs/8.11/ecs-log.html).
For instance, it allows to have an attribute with key `log.file`, that
will, thanks to `nlohmann::json`, appear as :
```
{
  "log": {
    "file": "xxx"
  }
}
```

Closes open-telemetry#3091

exporters/elasticsearch/src/es_log_recordable.cc

@@ -313,7 +313,7 @@ void ElasticSearchRecordable::SetAttribute(
     nostd::string_view key,
     const opentelemetry::common::AttributeValue &value) noexcept
 {
-  WriteKeyValue(key, value, "attributes");
+  WriteValue(value, key.data());
 }
 
 void ElasticSearchRecordable::SetResource(