tagging inconsistencies

The tagging concept and the `tags` field usage would benefit some thoughts and a properly defined API for the future (6.0?).

Some problems have been identified in #6142 and are being solved in #6177.

There is one inconsistency issue I'd like to discuss now to see if this is something we should fix in a minor/bugfix release.

- the `Event#tag` / `Event.tag()` methods deduplicates the `tags` field list, meaning that it will only insert the given tag in the `tags` field list only if it is not already in that list.

but

- the config `add_tag` directive does not do deduplication. The same tag can be inserted in the `tags` field multiple times.

Should we also do deduplication in the config `add_tag` ? if so, are there chances this break current configurations?

Otherwise, the larger problem here is that the `tags` field has never been properly considered a "reserved" field with a "defined" API, like `@timestamp` for example. so it is totally possible for anyone to place arbitrary values in the `tags` field but on the other hand, a lot of plugins rely on the existence and "validity" of the `tags` field.

I'd like open up the discussion on the best way to address this moving forward. Is it to simply define a new `@tags` field with a clear API? this would require changes is configurations? code sweep across plugins would also be necessary.

Start by comparing the deduplication behavior of Event#tag/Event.tag() with the config add_tag directive, then review the related discussion in #6142 and #6177. Done requires agreement on whether duplicate tags should be removed and on a clearly defined tags API, including the configuration and plugin impact.

Written by the indexing model from the issue text.