A health middleware

Start http handler on port 8080 and return OK. Also add some

documentation fixes for the prometheus middleware.

middleware/health/README.md

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+# health
+
+This module enables a simple health check.
+
+By default it will listen on port 9180.
+
+Restarting CoreDNS will stop the health checking. This is a bug. Also [this upstream
+Caddy bug](https://github.com/mholt/caddy/issues/675).
+
+## Syntax
+
+~~~
+health
+~~~
+
+It optionally takes an address, the default is `:8080`. The health path is fixed to `/health`. It
+will just return "OK", when CoreDNS is healthy.
+
+This middleware only needs to be enabled once.
+
+## Examples

middleware/health/health.go

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+package health
+
+import (
+	"io"
+	"log"
+	"net/http"
+	"sync"
+)
+
+var once sync.Once
+
+type Health struct {
+	Addr string
+}
+
+func health(w http.ResponseWriter, r *http.Request) {
+	io.WriteString(w, ok)
+}
+
+func (h Health) ListenAndServe() error {
+	if h.Addr == "" {
+		h.Addr = defAddr
+	}
+	once.Do(func() {
+		http.HandleFunc("/health", health)
+		go func() {
+			if err := http.ListenAndServe(h.Addr, nil); err != nil {
+				log.Printf("[ERROR] Failed to start health handler: %s", err)
+			}
+		}()
+	})
+	return nil
+}
+
+const (
+	ok      = "OK"
+	defAddr = ":8080"
+)