config/sys.config: increase StatsD reporter interval to 10s
When running ttcn3-s1gw-test locally, I noticed OsmoS1GW consuming 30-40% of a CPU core while idle (not serving any eNBs). Profiling with etop pointed to `exometer_report_statsd` as the culprit: with ~1720 metrics registered, it was sending ~1720 datagrams per second.
A 1-second reporting interval causes noticeable CPU overhead as the number of active metrics grows. With per-eNB and per-MME counters, the metric set scales with the number of connected eNBs/MMEs.
Increase the default StatsD reporting interval from 1s to 10s to reduce that overhead. Update the documentation accordingly.
