prometheus: add docs and fix varz a little

middleware/prometheus/README.md

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+# 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