Count of desktop app users over time

[bajtos]

Visualise the number of Station Desktop users over time. Right now, we have only a pie chart with the latest snapshot.

[bajtos]

We are visualising the current number of active Station Desktop in the Station Public Dashboard at https://github.com/filecoin-station/spark-stats/blob/7a7e7a72e875b166922e51c21b085bc5a6b8f736/observer/lib/observer.js#L100-L131

We want to add history for that data.

The implementation can follow the patterns used to collect & show Spark Retrieval Result Codes. See https://github.com/filecoin-station/spark-stats/blob/7a7e7a72e875b166922e51c21b085bc5a6b8f736/observer/lib/observer.js#L100-L131

I think the best granularity is to collect the data on a daily basis.

[pyropy]

I see that we have already an endpoint for selecting daily stations count. Could we use that instead of adding new observer loop?