plugin/metrics: add panic counter (#1778)

Count and export number of panics we see.

Fixes #1294

core/dnsserver/server.go

@@ -208,6 +208,7 @@ func (s *Server) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 			// In case the user doesn't enable error plugin, we still
 			// need to make sure that we stay alive up here
 			if rec := recover(); rec != nil {
+				vars.Panic.Inc()
 				DefaultErrorFunc(ctx, w, r, dns.RcodeServerFailure)
 			}
 		}()