add user defined attributes in _cat/nodes output

[bookshelfdave]

This PR allows custom node attributes to be displayed via /_cat/nodes.

Because custom node attributes are required to trigger this feature, rest-api-spec doesn't seem to satisfy the testing requirement. What's the best way to test this functionality, and where is a good place for me to start looking?

Here's a sample of the output when running with the following modifications to elasticsearch.yml:

node.rack: rack314
node.azone: us-east-1

Sample output:

$ curl -XGET 'http://localhost:9200/_cat/nodes'
host    ip          heap.percent ram.percent load node.role master name
epsilon 192.168.1.8            1          72 3.10 d         *      Alexander Bont
$ curl -XGET 'http://localhost:9200/_cat/nodes?h=node.rack,node.azone'
node.rack node.azone
  rack314  us-east-1

Closes #8000

[jasontedor]

This LGTM, but can you add a REST test in elasticsearch/rest-api-spec?

[dakrone]

I don't think we currently have a way to add node attributes to the nodes started for the REST tests, @clintongormley do you know if that's currently possible?

[drewr]

Apologies @metadave, I just added a comment to #8000 about the design here. Would you mind taking a look before merging?

[jasontedor]

@dakrone is correct, it is not currently possible. I investigated the possibility of adding it and I do not think it's worthwhile. The underlying issue is that to enable this functionality, we would no longer be able to reuse the cluster across REST tests (i.e., switch to Scope.TEST from Scope.SUITE). This would take the REST tests from slow to unacceptably slow.

[bookshelfdave]

I'm going to close this and work on a new API _cat/nodeattrs that has denormalized attributes. Thanks!