plugin/grpc: New gRPC plugin (#2667)

* plugin/grpc: New gRPC plugin

* some changes after the first review:

- remove healthcheck. gRPC already has this implicitly implemented
- some naming and stetic changes
- fix some comments
- other minor fixes

* plugin/grpc: New gRPC plugin

* some changes after the first review:

- remove healthcheck. gRPC already has this implicitly implemented
- some naming and stetic changes
- fix some comments
- other minor fixes

* add OWNERS file and change plugin order

* remove Rcode checker

plugin/grpc/metrics.go

@@ -0,0 +1,30 @@
+package grpc
+
+import (
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Variables declared for monitoring.
+var (
+	RequestCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "grpc",
+		Name:      "request_count_total",
+		Help:      "Counter of requests made per upstream.",
+	}, []string{"to"})
+	RcodeCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "grpc",
+		Name:      "response_rcode_count_total",
+		Help:      "Counter of requests made per upstream.",
+	}, []string{"rcode", "to"})
+	RequestDuration = prometheus.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "grpc",
+		Name:      "request_duration_seconds",
+		Buckets:   plugin.TimeBuckets,
+		Help:      "Histogram of the time each request took.",
+	}, []string{"to"})
+)