Rename the tool/completion_metrics directory

The directory contains tools related to code completion that are not
used to measure completion metrics, so the old name was too specific.

Change-Id: Ie8745901655df6e7b29da645b3b0a48011ca9f10
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174483
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/test/tool/completion_metrics/metrics_util_test.dart

@@ -4,7 +4,7 @@
 
 import 'package:test/test.dart';
 
-import '../../../tool/completion_metrics/metrics_util.dart';
+import '../../../tool/code_completion/metrics_util.dart';
 
 void main() {
   group('ArithmeticMeanComputer', () {