add meta packet statistics (#230)

This change add more metrics around "meta" (non "message" type packets).
For lighthouse packets, we also record statistics around the specific
lighthouse meta type.

We don't keep statistics for the "message" type so that we don't slow
down the fast path (and you can just look at metrics on the tun
interface to find that information).

examples/config.yml

@@ -177,6 +177,15 @@ logging:
   #subsystem: nebula
   #interval: 10s
 
+  # enables counter metrics for meta packets
+  #   e.g.: `messages.tx.handshake`
+  # NOTE: `message.{tx,rx}.recv_error` is always emitted
+  #message_metrics: false
+
+  # enables detailed counter metrics for lighthouse packets
+  #   e.g.: `lighthouse.rx.HostQuery`
+  #lighthouse_metrics: false
+
 # Handshake Manger Settings
 #handshakes:
   # Total time to try a handshake = sequence of `try_interval * retries`