middleware/metrics: cleanup (#355)

* middleware/metrics: add more metrics

middleware/cache:
Add metrics for number of elements in the cache. Also export the total
size. Update README to detail the new metrics.

middleware/metrics

Move metrics into subpackage called "vars". This breaks the import
cycle and is cleaner. This allows vars.Report to be used in the
the dnsserver to log refused queries.

middleware/metrics: tests

Add tests to the metrics framework. The metrics/test subpackage allows
scraping of the local server. Do a few test scrape of the metrics that
are defined in the metrics middleware.

This also allows metrics integration tests to check if the caching and
dnssec middleware export their metrics correctly.

* update README

* typos

* fix tests

core/dnsserver/config.go

@@ -41,3 +41,19 @@ func GetConfig(c *caddy.Controller) *Config {
 	ctx.saveConfig(c.Key, &Config{})
 	return GetConfig(c)
 }
+
+// GetMiddleware returns the middleware handler that has been added to the config under name.
+// This is useful to inspect if a certain middleware is active in this server.
+// Note that this is order dependent and the order is defined in directives.go, i.e. if your middleware
+// comes before the middleware you are checking; it will not be there (yet).
+func GetMiddleware(c *caddy.Controller, name string) middleware.Handler {
+	// TODO(miek): calling the handler h(nil) should be a noop...
+	conf := GetConfig(c)
+	for _, h := range conf.Middleware {
+		x := h(nil)
+		if name == x.Name() {
+			return x
+		}
+	}
+	return nil
+}