Support for data streams Elastic 7.9

[orjan]

Does this issue relate to a new feature or an existing bug?

• New Feature

What version of Serilog.Sinks.Elasticsearch is affected? Please list the related NuGet package.
<PackageReference Include="serilog.sinks.elasticsearch" Version="8.2.0" />

What is the target framework and operating system? See target frameworks & net standard matrix.

• netCore 3.1.402

Please describe the current behavior?
Data streams is a reliable way to minimize the number of shards in a cluster. Previously you had to setup an ILM, template and a write alias, data streams is more or less the same but endorsed by Elastic an transparent in Kibana and easier to configure.

There a constraint within data stream and it will only support the op_type=create
https://www.elastic.co/guide/en/elasticsearch/reference/master/use-a-data-stream.html#data-streams-bulk-indexing-requests

When trying to index documents from serilog the request looks like this

POST _bulk
{"index":{"_index":"my-data-stream","_type":"_doc"}}
{"@timestamp":"2020-09-11T16:14:17.6858199+02:00","level":"Error","messageTemplate":"Hello data stream from Serilog","message":"Hello data stream from Serilog"}

#! Deprecation: [types removal] Specifying types in bulk requests is deprecated.
{
  "took" : 0,
  "errors" : true,
  "items" : [
    {
      "index" : {
        "_index" : "my-data-stream",
        "_type" : "_doc",
        "_id" : null,
        "status" : 400,
        "error" : {
          "type" : "illegal_argument_exception",
          "reason" : "only write ops with an op_type of create are allowed in data streams"
        }
      }
    }
  ]
}

Please describe the expected behavior?
In order to support data streams we'll need to be able to alter the op_type from index to create then it should work:
https://github.com/serilog/serilog-sinks-elasticsearch/blob/master/src/Serilog.Sinks.Elasticsearch/Sinks/ElasticSearch/ElasticSearchSink.cs#L156-L160

There's also a deprecation warning for the _type removal but that's covered by a separate issue: #345

From an API point of view I can imagine specifying the op_type similar to this?

                .WriteTo.Elasticsearch(new ElasticsearchSinkOptions(new Uri("http://localhost:9200"))
                {
                    IndexFormat = "my-data-stream",
                    AutoRegisterTemplate = false,
                    BatchOpType = OpType.Create, 

[mivano]

Merged to dev. Closing this issue for now

[glacasa]

Just adding a note here, because I found this issue while trying to understand how to write in a DataStream.

If somebody is searching how to set the "BatchOpType", the property is called BatchAction

                  .WriteTo.Elasticsearch(new ElasticsearchSinkOptions(uri)
                  {
                      IndexFormat = "my-data-stream",
                      BatchAction= ElasticOpType.Create
                  });

[gustavodaquino]

For who that wants to run the code above in a ElasticSearch >= 8.0.0, remember to temporarily TypeName=null as referenced here.