plugin/view: Advanced routing interface and new 'view' plugin (#5538)

* introduce new interface "dnsserver.Viewer", that allows a plugin implementing it to decide if a query should be routed into its server block.
* add new plugin "view", that uses the new interface to enable a user to define expression based conditions that must be met for a query to be routed to its server block.

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/cache/setup.go

@@ -23,6 +23,12 @@ func setup(c *caddy.Controller) error {
 	if err != nil {
 		return plugin.Error("cache", err)
 	}
+
+	c.OnStartup(func() error {
+		ca.viewMetricLabel = dnsserver.GetConfig(c).ViewName
+		return nil
+	})
+
 	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
 		ca.Next = next
 		return ca