perf(metrics): implement plugin chain tracking (#7791)

Remove expensive runtime.Caller calls from metrics Recorder.WriteMsg
by tracking the responding plugin through the plugin chain instead.

- Add PluginTracker interface and pluginWriter wrapper in plugin.go
- Modify NextOrFailure to wrap ResponseWriter with plugin name
- Update metrics Recorder to implement PluginTracker
- Remove authoritativePlugin method using filepath inspection

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/metrics/recorder.go

@@ -1,8 +1,6 @@
 package metrics
 
 import (
-	"runtime"
-
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 
 	"github.com/miekg/dns"
@@ -11,8 +9,9 @@ import (
 // Recorder is a dnstest.Recorder specific to the metrics plugin.
 type Recorder struct {
 	*dnstest.Recorder
-	// CallerN holds the string return value of the call to runtime.Caller(N+1)
-	Caller [3]string
+	// Plugin holds the name of the plugin that wrote the response.
+	// This is set automatically by the plugin chain via the PluginTracker interface.
+	Plugin string
 }
 
 // NewRecorder makes and returns a new Recorder.
@@ -21,8 +20,15 @@ func NewRecorder(w dns.ResponseWriter) *Recorder { return &Recorder{Recorder: dn
 // WriteMsg records the status code and calls the
 // underlying ResponseWriter's WriteMsg method.
 func (r *Recorder) WriteMsg(res *dns.Msg) error {
-	_, r.Caller[0], _, _ = runtime.Caller(1)
-	_, r.Caller[1], _, _ = runtime.Caller(2)
-	_, r.Caller[2], _, _ = runtime.Caller(3)
 	return r.Recorder.WriteMsg(res)
 }
+
+// SetPlugin implements the plugin.PluginTracker interface.
+func (r *Recorder) SetPlugin(name string) {
+	r.Plugin = name
+}
+
+// GetPlugin implements the plugin.PluginTracker interface.
+func (r *Recorder) GetPlugin() string {
+	return r.Plugin
+}