Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/proxy/metrics.go

@@ -0,0 +1,30 @@
+package proxy
+
+import (
+	"sync"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Metrics the proxy plugin exports.
+var (
+	RequestDuration = prometheus.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: plugin.Namespace,
+		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{"proto", "proxy_proto", "from"})
+)
+
+// OnStartupMetrics sets up the metrics on startup. This is done for all proxy protocols.
+func OnStartupMetrics() error {
+	metricsOnce.Do(func() {
+		prometheus.MustRegister(RequestDuration)
+	})
+	return nil
+}
+
+var metricsOnce sync.Once