MINOR: sample: add us/ms support to date/http_date

MINOR: sample: add us/ms support to date/http_date

It can be sometimes interesting to have a timestamp with a
resolution of less than a second.
It is currently painful to obtain this, because concatenation
of date and date_us lead to a shorter timestamp during first
100ms of a second, which is not parseable and needs ugly ACLs
in configuration to prepend 0s when needed.
To improve this, add an optional <unit> parameter to date sample
to report an integer with desired unit.
Also support this unit in http_date converter to report
a date string with sub-second precision.