Support targeting multiple Elasticsearch indices

[faec]

Currently the shipper can only output to a single Elasticsearch index. This is in part a constraint of our support library go-elasticsearch, whose BulkIndexer can only target one index (thus to target more than one would require multiple bulk indexers, each with their own workers and so on). In the long run, however, we need to be able to ingest to multiple indices from a single worker / memory pool.

Resolving this will likely involve collaborating with the go-elasticsearch maintainers to add multi-index support to the library, or to find a suitable workaround for our purposes if that is infeasible for some reason. (It's also possible that we've misunderstood the API and this is already possible with the current library; either way, coordinating with @Anaethelion will probably clarify things.)

[Anaethelion]

You should be able to specify an Index field per BulkIndexerItem which serializes as the _index metadata, it overrides the default index specified at the root of the indexer.
See this integration test for reference.

Let me know if I can be of help!

[cmacknz]

As part of this we should upgrade to v8.6.0 of the Elasticsearch client to pick up the fix to elastic/go-elasticsearch#557