Insert data before the current time

__Proposal:__
Short summary of the feature.

There is a scenario where the results of the analysis of big data, but the execution time of this task is very long, the analysis report/results of this task will be stored in influxdb for easy business retrieval, but I found that if you specify a timestamp (most of them are before the current time) , The time written to influxdb is all from the current time.

__Current behavior:__
Describe what currently happens.

__Desired behavior:__
Describe what you want.
I think it is possible to specify a timestamp instead of the default writing time

__Alternatives considered:__
For example, logstash reads the time format in the log and replaces it with the timestamp stored in the ES

__Use case:__
Why is this important (helps with prioritizing requests)?

__Version__
influxdb 2.0.1 rest api

__Detail__
pastman

influxdb dasbboard

No repository file or test is named. Start by reproducing the InfluxDB 2.0.1 REST API write with a historical timestamp and compare the stored timestamp with the request. Done means writes can preserve an explicitly supplied timestamp instead of always using the current time.

Written by the indexing model from the issue text.