Metrics (#1579)

* plugin/metrics: set server address in context

Allow cross server block metrics to co-exist; for this we should label
each metric with the server label. Put this information in the context
and provide a helper function to get it out.

Abstracting with entirely away with difficult as the release client_go
(0.8.0) doesn't have the CurryWith functions yet. So current use is like
so:

define metric, with server label:

	RcodeCount = prometheus.NewCounterVec(prometheus.CounterOpts{
		Namespace: plugin.Namespace,
		Subsystem: "forward",
		Name:      "response_rcode_count_total",
		Help:      "Counter of requests made per upstream.",
	}, []string{"server", "rcode", "to"})

And report ith with the helper function metrics.WithServer:

	RcodeCount.WithLabelValues(metrics.WithServer(ctx), rc, p.addr).Add(1)

4 files changed, 43 insertions, 7 deletions

diff --git a/core/dnsserver/server.go b/core/dnsserver/server.go
index 2fa67be05..dbc7bd5ce 100644
--- a/core/dnsserver/server.go
+++ b/core/dnsserver/server.go
@@ -245,6 +245,11 @@ func (s *Server) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 		}
 
 		if h, ok := s.zones[string(b[:l])]; ok {
+
+			// Set server's address in the context so plugins can reference back to this,
+			// This will makes those metrics unique.
+			ctx = context.WithValue(ctx, plugin.ServerCtx{}, s.Addr)
+
 			if r.Question[0].Qtype != dns.TypeDS {
 				if h.FilterFunc == nil {
 					rcode, _ := h.pluginChain.ServeDNS(ctx, w, r)
@@ -287,6 +292,10 @@ func (s *Server) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 
 	// Wildcard match, if we have found nothing try the root zone as a last resort.
 	if h, ok := s.zones["."]; ok && h.pluginChain != nil {
+
+		// See comment above.
+		ctx = context.WithValue(ctx, plugin.ServerCtx{}, s.Addr)
+
 		rcode, _ := h.pluginChain.ServeDNS(ctx, w, r)
 		if !plugin.ClientWrite(rcode) {
 			DefaultErrorFunc(w, r, rcode)
@@ -359,11 +368,11 @@ const (
 	maxreentries = 10
 )
 
-// Key is the context key for the current server
-type Key struct{}
-
-// loopKey is the context key for counting self loops
-type loopKey struct{}
+type (
+	// Key is the context key for the current server
+	Key     struct{}
+	loopKey struct{} // loopKey is the context key for counting self loops
+)
 
 // enableChaos is a map with plugin names for which we should open CH class queries as
 // we block these by default.
diff --git a/plugin/metrics/context.go b/plugin/metrics/context.go
new file mode 100644
index 000000000..4cce871f5
--- /dev/null
+++ b/plugin/metrics/context.go
@@ -0,0 +1,24 @@
+package metrics
+
+import (
+	"github.com/coredns/coredns/plugin"
+
+	"golang.org/x/net/context"
+)
+
+// WithServer returns the current server handling the request. It returns the
+// server listening address: <scheme>://[<bind>]:<port> Normally this is
+// something like "dns://:53", but if the bind plugin is used, i.e. "bind
+// 127.0.0.53", it will be "dns://127.0.0.53:53", etc. If not address is found
+// the empty string is returned.
+//
+// Basic usage with a metric:
+//
+// <metric>.WithLabelValues(metrics.WithServer(ctx), labels..).Add(1)
+func WithServer(ctx context.Context) string {
+	srv := ctx.Value(plugin.ServerCtx{})
+	if srv == nil {
+		return ""
+	}
+	return srv.(string)
+}
diff --git a/plugin/metrics/metrics.go b/plugin/metrics/metrics.go
index a49984b3d..e16749fc0 100644
--- a/plugin/metrics/metrics.go
+++ b/plugin/metrics/metrics.go
@@ -12,6 +12,7 @@ import (
 	"github.com/coredns/coredns/coremain"
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/metrics/vars"
+
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
diff --git a/plugin/plugin.go b/plugin/plugin.go
index 2510dbaa6..b3d5cc6ab 100644
--- a/plugin/plugin.go
+++ b/plugin/plugin.go
@@ -68,8 +68,7 @@ func (f HandlerFunc) Name() string { return "handlerfunc" }
 // Error returns err with 'plugin/name: ' prefixed to it.
 func Error(name string, err error) error { return fmt.Errorf("%s/%s: %s", "plugin", name, err) }
 
-// NextOrFailure calls next.ServeDNS when next is not nill, otherwise it will return, a ServerFailure
-// and a nil error.
+// NextOrFailure calls next.ServeDNS when next is not nill, otherwise it will return, a ServerFailure and a nil error.
 func NextOrFailure(name string, next Handler, ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) { // nolint: golint
 	if next != nil {
 		if span := ot.SpanFromContext(ctx); span != nil {
@@ -107,3 +106,6 @@ var TimeBuckets = prometheus.ExponentialBuckets(0.00025, 2, 16) // from 0.25ms t
 
 // ErrOnce is returned when a plugin doesn't support multiple setups per server.
 var ErrOnce = errors.New("this plugin can only be used once per Server Block")
+
+// ServerCtx is the context key to pass server address context to the plugins handling the request.
+type ServerCtx struct{}