Collects broadcast packet metrics (#22)

The original DISCOv1 used to collect both broadcast and multicast
metrics, but for some reason I chose not to collect those for DISCOv2.
Upon closer inspection, we decided that while multicast packet metrics
were indeed not necessary, that broadcast packet metrics could possible
be useful. This commit adds collection for ingress and egress broadcast
packets.

metrics.yaml

@@ -38,3 +38,13 @@
   oidStub: .1.3.6.1.2.1.2.2.1.19
   mlabUplinkName: switch.discards.uplink.tx
   mlabMachineName: switch.discards.local.tx
+- name: ifHCInBroadcastPkts
+  description: Ingress broadcast packets.
+  oidStub: .1.3.6.1.2.1.31.1.1.1.9
+  mlabUplinkName: switch.broadcast.uplink.rx
+  mlabMachineName: switch.broadcast.local.rx
+- name: ifHCOutBroadcastPkts
+  description: Egress broadcast packets.
+  oidStub: .1.3.6.1.2.1.31.1.1.1.13
+  mlabUplinkName: switch.broadcast.uplink.tx
+  mlabMachineName: switch.broadcast.local.tx