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/vars/vars.go

@@ -0,0 +1,69 @@
+package vars
+
+import (
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Request* and Response* are the prometheus counters and gauges we are using for exporting metrics.
+var (
+	RequestCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "request_count_total",
+		Help:      "Counter of DNS requests made per zone, protocol and family.",
+	}, []string{"zone", "proto", "family"})
+
+	RequestDuration = prometheus.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		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{"zone"})
+
+	RequestSize = prometheus.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "request_size_bytes",
+		Help:      "Size of the EDNS0 UDP buffer in bytes (64K for TCP).",
+		Buckets:   []float64{0, 100, 200, 300, 400, 511, 1023, 2047, 4095, 8291, 16e3, 32e3, 48e3, 64e3},
+	}, []string{"zone", "proto"})
+
+	RequestDo = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "request_do_count_total",
+		Help:      "Counter of DNS requests with DO bit set per zone.",
+	}, []string{"zone"})
+
+	RequestType = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "request_type_count_total",
+		Help:      "Counter of DNS requests per type, per zone.",
+	}, []string{"zone", "type"})
+
+	ResponseSize = prometheus.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "response_size_bytes",
+		Help:      "Size of the returned response in bytes.",
+		Buckets:   []float64{0, 100, 200, 300, 400, 511, 1023, 2047, 4095, 8291, 16e3, 32e3, 48e3, 64e3},
+	}, []string{"zone", "proto"})
+
+	ResponseRcode = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "response_rcode_count_total",
+		Help:      "Counter of response status codes.",
+	}, []string{"zone", "rcode"})
+)
+
+const (
+	subsystem = "dns"
+
+	// Dropped indicates we dropped the query before any handling. It has no closing dot, so it can not be a valid zone.
+	Dropped = "dropped"
+)