Instrumentation doesn't work with esutil.bulkindexer

[lucax88x]

Ciao!

Calls using the default client, which I've properly configured, like the ping, is correctly sending traces to otel, but if we use the esutil.bulkindexer nothing is sent out.

	client, err := db.NewElasticsearchClient(db.ElasticsearchOptions{
		Address:  cb.Config.Elasticsearch.Address,
		Username: cb.Config.Elasticsearch.Username,
		Password: cb.Config.Elasticsearch.Password,
		Instrumentation: elasticsearch.NewOpenTelemetryInstrumentation(
			otel.GetTracerProvider(),
			cb.Config.Elasticsearch.CaptureSearchBody,
		),
	})

We found out that the following snippet of code, when it creates the esapi.BulkRequest, it's not setting the instrument field (I can guess because it's private...)

go-elasticsearch/esutil/bulk_indexer.go

Line 537 in 98a819b

req := esapi.BulkRequest{

Could it be the culript?
We are able to do a PR, if you can give us some insights how you would prefer to fix it.