Add a Plugin#id that return an ID for a plugin

[ph]

This method return an ID for the plugins and be configured by the users
in the configuration like this:

elasticsearch {
  id => "ABC"
  ...
}

This information will be used when collecting metrics for a specific
plugin. Allowing the user to change it allow to stick between restart.

Fixes #3892

[ph]

@jordansissel So I changed the logic a bit of the #id

By default if the user has not configured anything we will return a UUID.
The user can override this to return something more meaning for them.

[purbon]

I guess this will need some rebase here.

On the other side, I was not able to see the change for inputs? is there other system for inputs?

for the rest, it LGTM so happy to be merged.

[ph]

The inputs are covered higher directly in the pipeline.

[elasticsearch-bot]

Pier-Hugues Pellerin merged this into the following branches!

Branch	Commits
feature/metrics	1a780ab, ac2e030