Timestamp for field `timestamp`

[jalogisch]

Context

When you want to replace the field timestamp with a date from within the event you need to format that in a specific way.

Expected Content

The examples here might/should pick that topic up:

https://docs.graylog.org/en/4.0/pages/pipelines/functions.html#overview

or the format_date function should have that mentioned: https://docs.graylog.org/en/4.0/pages/pipelines/functions.html#format-date

The format that is needed is almost iso8601 ...

https://github.com/Graylog2/graylog2-server/blob/1a44cce49d74c59401eb5d2051810d64fda89f1d/graylog2-server/src/main/java/org/graylog2/plugin/Tools.java#L73