diff --git a/prometheus/metric.go b/prometheus/metric.go
index 9d9b81a..592eec3 100644
--- a/prometheus/metric.go
+++ b/prometheus/metric.go
@@ -108,15 +108,23 @@ func BuildFQName(namespace, subsystem, name string) string {
 	if name == "" {
 		return ""
 	}
-	switch {
-	case namespace != "" && subsystem != "":
-		return strings.Join([]string{namespace, subsystem, name}, "_")
-	case namespace != "":
-		return strings.Join([]string{namespace, name}, "_")
-	case subsystem != "":
-		return strings.Join([]string{subsystem, name}, "_")
+
+	sb := strings.Builder{}
+	sb.Grow(len(namespace) + len(subsystem) + len(name) + 2)
+
+	if namespace != "" {
+		sb.WriteString(namespace)
+		sb.WriteString("_")
 	}
-	return name
+
+	if subsystem != "" {
+		sb.WriteString(subsystem)
+		sb.WriteString("_")
+	}
+
+	sb.WriteString(name)
+
+	return sb.String()
 }
 
 type invalidMetric struct {