Integrate metrics to logtransfer

Add a new class Statistics in logtransfer.py which is a thread process that regularly (currently daily) collects metrics/statistics and includes these into the logs from an Oasis such that they are eventually transferred to logstash on central.

The Statistics internally calls the info/ (FastAPI endpoint), collects the statistics and sends them to the logstash (proxy) server. It should be rather easy to include more statistics, e.g. queries to elastic search or mongodb).

• Prefix for the statistics -- currently a "stats_" is prefixed to each entry -- would it be better to add an identifying field to the json?
• Run only the Statistics service (i.e. without logstash proxy, and let this run on central -- this includes the central statistics in the same format to the ES index)

Closes #1705 (closed)

Related: #886 (closed)

(old issues #820 (closed) )