Ignore import_of_legacy_library_into_null_safe (#70115)

dev/benchmarks/metrics_center/lib/google_benchmark.dart

diff --git a/dev/benchmarks/metrics_center/lib/google_benchmark.dart b/dev/benchmarks/metrics_center/lib/google_benchmark.dart
index 9c7c2aa..3e68145 100644
--- a/dev/benchmarks/metrics_center/lib/google_benchmark.dart
+++ b/dev/benchmarks/metrics_center/lib/google_benchmark.dart
@@ -9,7 +9,7 @@
 import 'dart:convert';
 import 'dart:io';
 
-import 'package:metrics_center/src/common.dart';
+import 'package:metrics_center/src/common.dart'; // ignore: import_of_legacy_library_into_null_safe
 
 const String _kTimeUnitKey = 'time_unit';