cmd/geth: decent error message if metrics are disabled

cmd/geth/monitorcmd.go

@@ -79,7 +79,7 @@ func monitor(ctx *cli.Context) {
 		if len(list) > 0 {
 			utils.Fatalf("No metrics specified.\n\nAvailable:\n - %s", strings.Join(list, "\n - "))
 		} else {
-			utils.Fatalf("No metrics specified.\n\nNo metrics collected (--metrics)\n")
+			utils.Fatalf("No metrics collected by geth (--%s).\n", utils.MetricsEnabledFlag.Name)
 		}
 	}
 	sort.Strings(monitored)

core/chain_manager.go

@@ -23,7 +23,6 @@ import (
 	"github.com/ethereum/go-ethereum/pow"
 	"github.com/ethereum/go-ethereum/rlp"
 	"github.com/hashicorp/golang-lru"
-	"github.com/rcrowley/go-metrics"
 	"github.com/syndtr/goleveldb/leveldb"
 )