Migrate getProject().index calls to lookup index

[tvernum]

This migrates a number of calls of the form

metadata.getProject().getIndexSafe(index)

or

metadata.getProject().index(index)

to

metadata.indexMetadata(index)

or

metadata.findIndex(index)

[elasticsearchmachine]

Pinging @elastic/es-distributed-coordination (Team:Distributed Coordination)

[nielsbauman]

Left one optional comment, other than that LGTM.

[nielsbauman]

Nit: in this case, I'd argue the assertion loses its value because a non-existent index (or project) would have already thrown an exception.

[ywangd]

LGTM

Not for this PR: I wonder whether we should make these methods more consistent:

1. ProjectMetadata#index
2. ProjectMetadata#getIndexSafe
3. Metadata#findIndex
4. Metadata#indexMetadata

The issues I can see are:

• Inconsistent naming style, e.g. getter vs builder pattern
• Except 2, they aer not clear which ones are throwing on null value
• 1 returns null while 3 returns Optional
• 4 spells out the full indexMetadata despite the other 3 are also about index metadata

The result is that I have to check up their usages each time I need to use them or review a PR containing their usages. I think it would be great if they are renamed more consistently and more obviously.

[tvernum]

Not for this PR: I wonder whether we should make these methods more consistent

It's come up before, and I think it makes sense. I'm not planning to tackle it, but it would be good for someone else to do so.