Add ECS Compatibility mode

[yaauie]

Adds an ECS Compatibility mode.

Without this mode, metadata is persisted to each event's top-level host and path fields, but with ECS enabled, it is persisted to the ECS-compatible [host][name] and [log][file][path].

[karenzone]

Doc/changelog review: Builds and renders cleanly. Comments inline for your consideration

[jsvd]

Since the PR already has a review request for @kares I removed elasticsearch-bot as an assignee

[kaisecheng]

LGTM 🌈 code is clean. CI red is unrelated flaky test

[kares]

Late Review: 👍

there's some opportunity for a minor clean (as outlined in the comment),

also while keeping [@metadata][path] and [@metadata][host] makes sense for compatibility and smooth migration I wonder if in case of multiple inputs if it would have been worth also transitioning to a [@metadata][input][file][path] (and [@metadata][input][file][host]) to avoid potential collisions.

[kares]

would have been nice to move this final event manipulation part to the file input and simply have:

    def process_event(event)
      input.post_process_this(event, path) # added path argument
    end

... instead of setting meta-data just to retrieve it event.get('[@metadata][path]') right away in post_process_this