core: metrics collection for transaction events (#3157)

* core: Add metrics collection for transaction events; replace/discard for pending and future queues, as well as invalid transactions

* core: change namespace for txpool metrics

* core: define more metrics (not yet used)

* core: implement more tx metrics for when transactions are dropped

* core: minor formatting tweeks (will squash later)

* core: remove superfluous meter, fix missing pending nofunds

* core, metrics: switch txpool meters to counters

metrics/metrics.go

@@ -48,6 +48,15 @@ func init() {
 	exp.Exp(metrics.DefaultRegistry)
 }
 
+// NewCounter create a new metrics Counter, either a real one of a NOP stub depending
+// on the metrics flag.
+func NewCounter(name string) metrics.Counter {
+	if !Enabled {
+		return new(metrics.NilCounter)
+	}
+	return metrics.GetOrRegisterCounter(name, metrics.DefaultRegistry)
+}
+
 // NewMeter create a new metrics Meter, either a real one of a NOP stub depending
 // on the metrics flag.
 func NewMeter(name string) metrics.Meter {