Igor Motov 001b78f704 Replace IndexMetaData.Custom with Map-based custom metadata (#32749)
This PR removes the deprecated `Custom` class in `IndexMetaData`, in favor
of a `Map<String, DiffableStringMap>` that is used to store custom index
metadata. As part of this, there is now no way to set this metadata in a
template or create index request (since it's only set by plugins, or dedicated
REST endpoints).

The `Map<String, DiffableStringMap>` is intended to be a namespaced `Map<String,
String>` (`DiffableStringMap` implements `Map<String, String>`, so the signature
is more like `Map<String, Map<String, String>>`). This is so we can do things
like:

``` java
Map<String, String> ccrMeta = indexMetaData.getCustom("ccr");
```

And then have complete control over the metadata. This also means any
plugin/feature that uses this has to manage its own BWC, as the map is just
serialized as a map. It also means that if metadata is put in the map that isn't
used (for instance, if a plugin were removed), it causes no failures the way
an unregistered `Setting` would.

The reason I use a custom `DiffableStringMap` here rather than a plain
`Map<String, String>` is so the map can be diffed with previous cluster state
updates for serialization.

Supersedes #32683
2018-08-30 13:57:00 -06:00
..
2018-08-29 17:43:40 +03:00