1/5 = 0.2 Extend the Sample entity to persist a sequence number.

[uliSchuster]

• As a user of time series data, I want to know if some samples are missing so that I can take remedial actions, like interpolation.
• As a a DevOps engineer tasked to debug problems with sample ingestion, with sensor nodes or with the transport network, I want to know if samples are missing to have a starting point for further investigations.

Currently, the Sample entity records a timestamp plus the measured quantities. From this information, it is not possible to conclude if samples are missing in a time series.

However, some sample nodes or sample networks do record sequence numbers for all recorded samples. When available, we should make this information part of each Sample entity and persist it.

Goal of the present issue is to extend the Sample entity and related API code so that a sequence number can be provided upon ingestion and retrieved via the Timeseries and Sample API.

Enhancing the TTN client is subject of a separate issue.