Clarify that the cluster-health operation's index parameter also accepts data streams

[michaelbaamonde]

The cluster-health operation accepts an optional index parameter. This seems to map to what the Elasticsearch documentation refers to as the target of the health check:

Comma-separated list of data streams, indices, and index aliases used to limit the request. Wildcard expressions (*) are supported.

So, it appears that the index parameter in Rally's cluster-health operation implicitly accepts data streams. We should at least clarify this in the docs in the short term.

It's also worth considering renaming the parameter to target. This would be a breaking change, however, so there's a cost-benefit analysis to be done. Curious what others think.

[danielmitterdorfer]

It's also worth considering renaming the parameter to target. This would be a breaking change, however, so there's a cost-benefit analysis to be done. Curious what others think.

I'm in favor of renaming, especially considering https://github.com/elastic/rally/issues/1359 we should aim for a consistent terminology. Renaming is usually not much of a problem because we can support both names for a grace period and emit a warning if the deprecated name is used.