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