Fix flaky restrict_gitlab_schema_spec.rb

This test was failing because `Feature.enabled?(:redis_hll_tracking)`
was called in the migration and expecting the `feature_names` database
table to be accessed. However, it wasn't accessed for two reasons:

1. In tests we stub out feature flag access and use an in-memory
cache. However, even though the test had `stub_feature_flags: false`
set in the metadata, a stale `Feature` was still memoized and had to be
cleared out.

2. With the feature flag stubs disabled, this enabled the Redis cache
for feature flags. We need to clear this out between tests to ensure
the database is hit.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/416143

lib/gitlab/metrics/exporter/base_exporter.rb

@@ -68,11 +68,8 @@ def stop_working
           if server
             # we close sockets if thread is not longer running
             # this happens, when the process forks
-            if thread.alive?
-              server.shutdown
-            else
-              server.listeners.each(&:close)
-            end
+            server.listeners.each(&:close) unless thread.alive?
+            server.shutdown
           end
           @server = nil

spec/lib/gitlab/database/migration_helpers/restrict_gitlab_schema_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 RSpec.describe Gitlab::Database::MigrationHelpers::RestrictGitlabSchema, query_analyzers: false,
-  stub_feature_flags: false, feature_category: :cell do
+  stub_feature_flags: false, use_clean_rails_redis_caching: true, feature_category: :cell do
   let(:schema_class) { Class.new(Gitlab::Database::Migration[1.0]).include(described_class) }
   # We keep only the GitlabSchemasValidateConnection analyzer running

spec/lib/gitlab/metrics/exporter/base_exporter_spec.rb

@@ -216,7 +216,7 @@ def call(env)
       # in separate thread
       allow_any_instance_of(::WEBrick::HTTPServer)
         .to receive(:start).and_wrap_original do |m, *args|
-        Thread.new do
+        @server_thread = Thread.new do # rubocop:disable RSpec/InstanceVariable -- let does not work for this case
           m.call(*args)
         rescue IOError
           # is raised as we close listeners
@@ -224,8 +224,17 @@ def call(env)
       end
     end
+    attr_reader :server_thread
+
     after do
       exporter.stop
+
+      next unless server_thread
+
+      server_thread.join(0.05)
+      raise '`exporter.stop` should terminate `server_thread`' if server_thread.alive?
+    ensure
+      server_thread.kill.join if server_thread
     end
     with_them do

spec/support/helpers/stub_feature_flags.rb

@@ -22,6 +22,7 @@ def stub_all_feature_flags
   def unstub_all_feature_flags
     Feature.stub = false
+    Feature.reset_flipper
   end
   # Stub Feature flags with `flag_name: true/false`