Migrate to use the published metrics_center (flutter#23767)

This change shouldn't affect anything except for the "Linux benchmarks" LUCI post-submit bot.

We'll closely monitor the LUCI bot to see if this works as intended. If so, we'll continue the migration in Cocoon (flutter/cocoon#1062) to cut the dependency on the old Cocoon datastore and Tong's desktop.

Related issue: flutter/flutter#73872

testing/benchmark/bin/parse_and_send.dart

@@ -7,8 +7,7 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:git/git.dart';
-import 'package:metrics_center/flutter.dart';
-import 'package:metrics_center/google_benchmark.dart';
+import 'package:metrics_center/metrics_center.dart';
 
 Future<String> _getGitRevision() async {
   final GitDir gitDir = await GitDir.fromExisting('../../');

testing/benchmark/pubspec.yaml

@@ -2,10 +2,8 @@ name: flutter_engine_benchmark
 
 dependencies:
   git: any
-  metrics_center:
-    # TODO(liyuqian): once metrics_center is properly reviewed, add it to
-    # flutter/packages, publish on pub.dev, and use the published package here.
-    git: https://github.com/liyuqian/metrics_center.git
+
+  metrics_center: 0.0.4
 
 dev_dependencies:
   test: any