Fix usage trends fixtures

This change prevents generating dummy data for the billable_users
metric in Usage Trends. Generating random values can cause problems when
adding a license in GDK.

Additional change: remove EXPERIMENTAL_IDENTIFIERS unused constant

app/models/analytics/usage_trends/measurement.rb

@@ -5,8 +5,6 @@ module Analytics
     class Measurement < ApplicationRecord
       self.table_name = 'analytics_instance_statistics_measurements'
 
-      EXPERIMENTAL_IDENTIFIERS = %i[pipelines_succeeded pipelines_failed pipelines_canceled pipelines_skipped].freeze
-
       enum identifier: {
         projects: 1,
         users: 2,

db/fixtures/development/29_usage_trends.rb

@@ -9,7 +9,8 @@ Gitlab::Seeder.quiet do
 
   model_class = Analytics::UsageTrends::Measurement
 
-  measurements = model_class.identifiers.flat_map do |_, id|
+  # Skip generating data for billable_users, to avoid license check problems
+  measurements = model_class.identifiers.except(:billable_users).each_value.flat_map do |id|
     recorded_at = 60.days.ago
     current_count = rand(1_000_000)