metrics: remove RR type (#4534)

To combat label cardinality explosions remove the type from metrics.
This was most severe in the histogram for request duration, remove it
there.

It's also highlighted difference between grpc and forward code, where
forward did use type and grpc didn't; getting rid of all that "fixes"
that discrepancy

Move monitor.go back into the vars directory and make it private again.
Also name it slightly better

Fixes: #4507

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/forward/metrics.go

@@ -27,7 +27,7 @@ var (
 		Name:      "request_duration_seconds",
 		Buckets:   plugin.TimeBuckets,
 		Help:      "Histogram of the time each request took.",
-	}, []string{"to", "rcode", "type"})
+	}, []string{"to", "rcode"})
 	HealthcheckFailureCount = promauto.NewCounterVec(prometheus.CounterOpts{
 		Namespace: plugin.Namespace,
 		Subsystem: "forward",