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/metrics/README.md

@@ -0,0 +1,53 @@
+# prometheus
+
+This module enables prometheus metrics for CoreDNS.
+
+The default location for the metrics is `localhost:9153`. The metrics path is fixed to `/metrics`.
+The following metrics are exported:
+
+* coredns_dns_request_count_total{zone, proto, family}
+* coredns_dns_request_duration_milliseconds{zone}
+* coredns_dns_request_size_bytes{zone, proto}
+* coredns_dns_request_do_count_total{zone}
+* coredns_dns_request_type_count_total{zone, type}
+* coredns_dns_response_size_bytes{zone, proto}
+* coredns_dns_response_rcode_count_total{zone, rcode}
+
+Each counter has a label `zone` which is the zonename used for the request/response.
+
+Extra labels used are:
+
+* `proto` which holds the transport of the response ("udp" or "tcp")
+* The address family (`family`) of the transport (1 = IP (IP version 4), 2 = IP6 (IP version 6)).
+* `type` which holds the query type. It holds most common types (A, AAAA, MX, SOA, CNAME, PTR, TXT,
+  NS, SRV, DS, DNSKEY, RRSIG, NSEC, NSEC3, IXFR, AXFR and ANY) and "other" which lumps together all
+  other types.
+* The `response_rcode_count_total` has an extra label `rcode` which holds the rcode of the response.
+
+If monitoring is enabled, queries that do not enter the plugin chain are exported under the fake
+name "dropped" (without a closing dot - this is never a valid domain name).
+
+
+## Syntax
+
+~~~
+prometheus [ADDRESS]
+~~~
+
+For each zone that you want to see metrics for.
+
+It optionally takes an address to which the metrics are exported; the default
+is `localhost:9153`. The metrics path is fixed to `/metrics`.
+
+## Examples
+
+Use an alternative address:
+
+~~~
+prometheus localhost:9253
+~~~
+
+# Bugs
+
+When reloading, we keep the handler running, meaning that any changes to the handler's address
+aren't picked up. You'll need to restart CoreDNS for that to happen.