Add _cat/nodeattrs API

[bookshelfdave]

This provides a _cat/nodeattrs API call, which presents custom node attributes in a denormalized table.

This was added in addition to the _cat/nodes endpoint as we don't want to create dynamic columns for each user defined attribute in _cat/nodes.

Closes #8000

cc @drewr

[nik9000]

Looks good to me. I'm surprised it takes three requests to get that but I don't really mind it.

[drewr]

Now that I'm seeing these, I wonder if the default names should be attr and value. We could add aliases if we need longer. Since it's such a small API it's probably fine with the short versions.

[drewr]

Other than small comment, LGTM, thanks @metadave!

@nik9000 That's the beauty of the cat API, it slices across the stats and info APIs internally so you don't have to! 😹

[bookshelfdave]

Updated with @drewr's suggestions. How do I go about getting a doc for http://www.elastic.co/guide/en/elasticsearch/reference/master/cat-nodeattrs.html ?

[nik9000]

How do I go about getting a doc for http://www.elastic.co/guide/en/elasticsearch/reference/master/cat-nodeattrs.html ?

Make one in docs/reference/cat I believe.

[bookshelfdave]

thanks @nik9000

[nik9000]

Do it as part of this pull request if you can! Its better to commit the docs at the same time.

This has documentation on how to build them: https://github.com/elastic/docs

[bookshelfdave]

Updated w/ asciidoc

[nik9000]

Do you need to add a link in the cat.asciidoc?

[bookshelfdave]

yes, indeed I do.

[dakrone]

Not sure if we want this optimization, but we could always run these three requests in parallel, putting the responses in three AtomicReferences and using three CountDownLatchs to wait until all three complete.

[drewr]

I implemented the early cat APIs that way and @kimchy wanted to nest instead. If the cluster's busy then you only have one dangling request.

[dakrone]

That works for me!

[bookshelfdave]

shall I squash and merge?

[dakrone]

Yep, LGTM, can you also add the v2.0.0 tag?