Change timestamp precision to ns

Hi @mariog

when testing the influxDB class, we realized that timestamps pushed into influxDB were in ms, while in the past they were in ns. We traced it back to a change in the external library that provides the C++ API to influxDB: https://github.com/orca-zhang/influxdb-cpp/commit/5ee1f8869f6a4b7d647a884f82c1551afa16358d

The consequence of this was that Grafana wasn't able to display any of the data point we took today.

If in the influxDB class instead we force the timestamp to be recorded as ns, we are again able to find the data point in Grafana.

We may want to think about the fact that we are depending on a library that can be changed at any time with no warning.

Cheers, Elisabetta