Metrics: expand coredns_dns_responses_total with plugin label

This adds (somewhat hacky?) code to add a plugin label to the
coredns_dns_responses_total metric. It's completely obvlious to the
plugin as we just check who called the *recorder.WriteMsg method. We use
runtime.Caller( 1 2 3) to get multiple levels of callers, this should be
deep enough, but it depends on the dns.ResponseWriter wrapping that's
occuring.

README.md of metrics updates and test added in test/metrics_test.go to
check for the label being set.

I went through the plugin to see what metrics could be removed, but
actually didn't find any, the plugin push out metrics that make sense.

Due to the path fiddling to figure out the plugin name I doubt this
works (out-of-the-box) for external plugins, but I haven't tested that.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/metrics/metrics.go

@@ -8,6 +8,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/coredns/caddy"
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/pkg/reuseport"
 
@@ -31,6 +32,8 @@ type Metrics struct {
 	zoneNames []string
 	zoneMap   map[string]struct{}
 	zoneMu    sync.RWMutex
+
+	plugins map[string]struct{} // all available plugins, used to determine which plugin made the client write
 }
 
 // New returns a new instance of Metrics with the given address.
@@ -39,6 +42,7 @@ func New(addr string) *Metrics {
 		Addr:    addr,
 		Reg:     prometheus.DefaultRegisterer.(*prometheus.Registry),
 		zoneMap: make(map[string]struct{}),
+		plugins: pluginList(caddy.ListPlugins()),
 	}
 
 	return met
@@ -140,6 +144,19 @@ func keys(m map[string]struct{}) []string {
 	return sx
 }
 
+// pluginList iterates over the returned plugin map from caddy and removes the "dns." prefix from them.
+func pluginList(m map[string][]string) map[string]struct{} {
+	pm := map[string]struct{}{}
+	for _, p := range m["others"] {
+		// only add 'dns.' plugins
+		if len(p) > 3 {
+			pm[p[4:]] = struct{}{}
+			continue
+		}
+	}
+	return pm
+}
+
 // ListenAddr is assigned the address of the prometheus listener. Its use is mainly in tests where
 // we listen on "localhost:0" and need to retrieve the actual address.
 var ListenAddr string