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, 1 insertions, 0 deletions

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"
 )