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/handler.go

@@ -2,6 +2,7 @@ package metrics
 
 import (
 	"context"
+	"path/filepath"
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/metrics/vars"
@@ -33,10 +34,32 @@ func (m *Metrics) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 		// see https://github.com/coredns/coredns/blob/master/core/dnsserver/server.go#L318
 		rc = status
 	}
-	vars.Report(WithServer(ctx), state, zone, rcode.ToString(rc), rw.Len, rw.Start)
+	plugin := m.authoritativePlugin(rw.Caller1, rw.Caller2, rw.Caller3)
+	vars.Report(WithServer(ctx), state, zone, rcode.ToString(rc), plugin, rw.Len, rw.Start)
 
 	return status, err
 }
 
 // Name implements the Handler interface.
 func (m *Metrics) Name() string { return "prometheus" }
+
+// authoritativePlugin returns which of made the write, if none is found the empty string is returned.
+func (m *Metrics) authoritativePlugin(a, b, c string) string {
+	// a b and c contain the full path of the caller, the plugin name 2nd last elements
+	// .../coredns/plugin/whoami/whoami.go --> whoami
+	// this is likely FS specific, so use filepath.
+	plug := filepath.Base(filepath.Dir(a))
+	if _, ok := m.plugins[plug]; ok {
+		return plug
+	}
+	plug = filepath.Base(filepath.Dir(b))
+	if _, ok := m.plugins[plug]; ok {
+		return plug
+	}
+	plug = filepath.Base(filepath.Dir(c))
+	if _, ok := m.plugins[plug]; ok {
+		return plug
+	}
+
+	return ""
+}