Retrieving _ttl rebases the expiration date against now() instead of the value of _timestamp

[jpountz]

When you index a document with a _ttl, what we store/index internally is the expiration date, ie. the sum of _timestamp and _ttl. However when we retrieve the value of the _ttl, we return the difference between the expiration date and now().

While this is useful to see in how long documents will be expired, it makes it impossible to reindex a document with the exact same expiration date. In my opinion, we should return the original value of the _ttl instead, since it is relative to the value of _timestamp anyway.

[jpountz]

It would be nice to have it fixed in 2.0 to make sure that all 2.0+ indices can be safely reindexed.

[s1monw]

++

[nithyanv]

+1

[nozari]

+1

[clintongormley]

Closing in favour of #18280