Adjust rails custom metrics generator to prefix filename with timestamp

doc/development/usage_ping/metrics_dictionary.md

@@ -67,6 +67,8 @@ tier: ['free', 'starter', 'premium', 'ultimate', 'bronze', 'silver', 'gold']
 
 The GitLab codebase provides a dedicated [generator](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/generators/gitlab/usage_metric_definition_generator.rb) to create new metric definitions.
 
+For uniqueness, the generated file includes a timestamp prefix, in ISO 8601 format.
+
 The generator takes the key path argument and 2 options and creates the metric YAML definition in corresponding location:
 
 - `--ee`, `--no-ee` Indicates if metric is for EE.

lib/generators/gitlab/usage_metric_definition_generator.rb

@@ -67,8 +67,11 @@ module Gitlab
       options[:dir]
     end
 
+    # Example of file name
+    #
+    # 20210201124931_g_project_management_issue_title_changed_weekly.yml
     def file_name
-      key_path.split('.').last
+      "#{Time.now.utc.strftime("%Y%m%d%H%M%S")}_#{key_path.split('.').last}"
     end
 
     def directory