[Change Proposal] Support look back time for TSDB indexes #1179

Time series data streams have a writable window of 2 hours in the past. This means that documents whose @timestamp is older than these 2 hours are rejected.

There are integrations use cases where data is collected in longer periods, for example metrics that are published only daily. In these cases the 2 hours window is too small.

Elasticsearch supports index.look_back_time to change this 2 hours default.

Allow to use index.look_back_time in the index settings. This should be only allowed when time series is enabled.
For completitude, allow to use also index.look_ahead_time, that increases the window in the future.

As a workaround users can already override this value using @custom index templates, but it should work out of the box for integrations that are known to collect data older than 2 hours.

Feature requested in elastic/integrations#19339 (comment).