middleware/proxy: implement Exchanger (#480)

By defining and using an proxy.Exchanger interface we make the proxy
more generic and we can then fold back httproxy into proxy.

This overrides #463 and #473 and should make futures extensions rather
trivial

* Add docs that talk about `protocol` and how to set it.
* middleware/proxy: rename New to NewLookup
  It's used as a Lookup mechanism not as a completely new proxy,
  reflect that in the name.
* Set maxfails to 3 by default when looking up names.

Most of the changes have been copied
from https://github.com/johnbelamaric/coredns/pull/1/files

1 files changed, 3 insertions, 5 deletions

diff --git a/middleware/proxy/metrics.go b/middleware/proxy/metrics.go
index f3112dd76..a2205c692 100644
--- a/middleware/proxy/metrics.go
+++ b/middleware/proxy/metrics.go
@@ -12,14 +12,14 @@ import (
 var (
 	RequestDuration = prometheus.NewHistogramVec(prometheus.HistogramOpts{
 		Namespace: middleware.Namespace,
-		Subsystem: subsystem,
+		Subsystem: "proxy",
 		Name:      "request_duration_milliseconds",
 		Buckets:   append(prometheus.DefBuckets, []float64{50, 100, 200, 500, 1000, 2000, 3000, 4000, 5000, 10000}...),
 		Help:      "Histogram of the time (in milliseconds) each request took.",
-	}, []string{"zone"})
+	}, []string{"protocol", "zone"})
 )
 
-// OnStartup sets up the metrics on startup.
+// OnStartup sets up the metrics on startup. This is done for all proxy protocols.
 func OnStartup() error {
 	metricsOnce.Do(func() {
 		prometheus.MustRegister(RequestDuration)
@@ -28,5 +28,3 @@ func OnStartup() error {
 }
 
 var metricsOnce sync.Once
-
-const subsystem = "proxy"