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)

1 files changed, 4 insertions, 2 deletions

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{}