roachtest: allow NoNodes to c.NewMonitor

It can be helpful to spin up a `Monitor` that monitors only a set of
goroutines, without also watching the nodes in underlying cluster,
which is the default, and which could not be "narrowed down" to an
empty set of nodes.

This commit permits that.

Release note: None

pkg/cmd/roachtest/cluster.go

@@ -2359,6 +2359,12 @@ func (c *clusterImpl) MakeNodes(opts ...option.Option) string {
 		if s, ok := o.(nodeSelector); ok {
 			r = s.Merge(r)
 		}
+		if _, ok := o.(*option.NoNodes); ok {
+			return ""
+		}
+		if _, ok := o.(option.NoNodes); ok {
+			return ""
+		}
 	}
 	return c.name + r.String()
 }

pkg/cmd/roachtest/monitor.go

@@ -180,6 +180,10 @@ func (m *monitorImpl) wait() error {
 			wg.Done()
 		}()
 
+		if m.nodes == "" {
+			<-m.ctx.Done()
+			return
+		}
 		messagesChannel, err := roachprod.Monitor(m.ctx, m.l, m.nodes, install.MonitorOpts{})
 		if err != nil {
 			setErr(errors.Wrap(err, "monitor command failure"))

pkg/cmd/roachtest/option/node_list_option.go

@@ -113,3 +113,9 @@ func (n NodeListOption) String() string {
 	}
 	return buf.String()
 }
+
+// NoNodes is an option that explicitly deselects all nodes.
+type NoNodes struct{}
+
+// Option implements option.Option.
+func (n NoNodes) Option() {}