Fix reporting of network activity on Linux

The current Kamon release doesn't seem to report network activity on
Linux, in docker or outside. This may be related with the
"updateAttributes()" method in OSHI not being invoked.

A local experiment confirmed that OSHI will indeed report an unchanging
value for e.g. `getBytesRecv()`, unless `updateAttributes()` is invoked
first.

Hence, this should probably get things working again.

instrumentation/kamon-system-metrics/src/main/scala/kamon/instrumentation/system/host/HostMetricsCollector.scala

@@ -221,6 +221,7 @@ class HostMetricsCollector(ec: ExecutionContext) extends ScheduledAction {
 
       interfaces.asScala.foreach(interface => {
         if (_settings.trackedInterfaces.accept(interface.getName)) {
+          interface.updateAttributes()
           val interfaceInstruments = _networkActivityInstruments.interfaceInstruments(interface.getName)
           interfaceInstruments.receivedBytes.diff(interface.getBytesRecv)
           interfaceInstruments.sentBytes.diff(interface.getBytesSent)