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, 24 insertions, 0 deletions

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)
+}