show example file for endpoint 'database_statistics'

(cherry picked from commit 1d0d89dc)

Co-authored-by: schroeder5 <s.schroeder@fz-juelich.de>

static/db_statistics.json

+{
+    "users": 0,
+    "stations": 23979,
+    "time-series": 432880,
+    "data records": 65637800808
+}
+