prometheus: add docs and fix varz a little

middleware/prometheus/README.md

+# prometheus
+
+This module enables prometheus metrics for CoreDNS.
+
+The following metrics are exported:
+
+* coredns_dns_request_count_total
+* coredns_dns_request_duration_seconds
+* coredns_dns_response_size_bytes
+* coredns_dns_response_rcode_count_total
+
+Each counter has a label `zone` which is the zonename used for the request/response.
+The `request_count` metrics has an extra label `qtype` which holds the qtype. And
+`rcode_count` has an extra label which has the rcode.
+
+Restarting CoreDNS will stop the monitoring. This is a bug. Also [this upstream
+Caddy bug](https://github.com/mholt/caddy/issues/675).
+
+## Syntax
+
+~~~
+prometheus
+~~~
+
+For each zone that you want to see metrics for.
+
+It optionally takes an address where the metrics are exported, the default
+is `localhost:9153`. The metrics path is fixed to `/metrics`.
+
+## Examples

middleware/prometheus/metrics.go

@@ -9,7 +9,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-const namespace = "daddy"
+const namespace = "coredns"
 
 var (
 	requestCount    *prometheus.CounterVec
@@ -74,7 +74,7 @@ func define(subsystem string) {
 	responseRcode = prometheus.NewCounterVec(prometheus.CounterOpts{
 		Namespace: namespace,
 		Subsystem: subsystem,
-		Name:      "rcode_code_count_total",
+		Name:      "response_rcode_count_total",
 		Help:      "Counter of response status codes.",
 	}, []string{"zone", "rcode"})
 }