go.mod: use go.etcd.io/etcd/v3 versioning

This change makes the etcd package compatible with the existing Go
ecosystem for module versioning.

Used this tool to update package imports:
  https://github.com/KSubedi/gomove

clientv3/example_metrics_test.go

@@ -23,7 +23,7 @@ import (
 	"net/http"
 	"strings"
 
-	"go.etcd.io/etcd/clientv3"
+	"go.etcd.io/etcd/v3/clientv3"
 
 	grpcprom "github.com/grpc-ecosystem/go-grpc-prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"