Allow to set metric timestamp from query results

[burningalchemist]

Discussed in #301

^{Originally posted by Koaxiel August 28, 2023}
Hello,

I would like to know your opinion on the addition of a feature to set the metric' timestamp according to the result of a query.
Indeed, the Prometheus format allows us to add the timestamp in addition to the value of a metric.
From the Golang client, the NewMetricWithTimestamp function allows to do that easily.
We can imagine having a new setting (like key_timestamp) to define the column to use to set the metric' timestamp.

I know it's not a common need, but it's useful when using DBs like Vertica to make heavy analyses and to be able to keep the initial timestamp (the one when the DB is injected) in the results.

WDYT?