Fix rake gitlab:setup failing on new installs

For source installs, running `rake gitlab:setup` will fail with this
error:

```

$ bundle exec rake gitlab:setup RAILS_ENV=production
rake aborted!
ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR: relation
"features" does not exist LINE 8: WHERE a.attrelid =
'"features"'::regclass
```

This happens because the setup Rake task does the following:

1. Checks that Gitaly servers are up
2. The gRPC message attempts to send the activated feature flags
in the metadata.
3. However, since the `features` table doesn't exist, this fails.

To fix this, we just return an empty set if neither the database nor the
`features` table exists.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/212759

changelogs/unreleased/sh-check-features-table-gitaly.yml

+---
+title: Fix rake gitlab:setup failing on new installs
+merge_request: 28270
+author:
+type: fixed

lib/feature/gitaly.rb

@@ -14,6 +14,9 @@ class Feature
       end
 
       def server_feature_flags
+        # We need to check that both the DB connection and table exists
+        return {} unless ::Gitlab::Database.cached_table_exists?(FlipperFeature.table_name)
+
         Feature.persisted_names
           .select { |f| f.start_with?(PREFIX) }
           .map do |f|

spec/lib/feature/gitaly_spec.rb

@@ -32,5 +32,15 @@ describe Feature::Gitaly do
 
     it { is_expected.to be_a(Hash) }
     it { is_expected.to eq("gitaly-feature-mep-mep" => "true") }
+
+    context 'when table does not exist' do
+      before do
+        allow(::Gitlab::Database).to receive(:cached_table_exists?).and_return(false)
+      end
+
+      it 'returns an empty Hash' do
+        expect(subject).to eq({})
+      end
+    end
   end
 end