Fix alt_usage_data to properly return fallback in specs

This changes stubbing #alt_usage_data method to properly return fallback
data value - blindly stubbing as `-1` failed specs for certain JSON
schemas.

ee/spec/config/metrics/every_metric_definition_spec.rb

@@ -75,7 +75,10 @@ RSpec.describe 'Every metric definition' do
   end
 
   before do
-    allow(Gitlab::UsageData).to receive_messages(count: -1, distinct_count: -1, estimate_batch_distinct_count: -1, sum: -1, alt_usage_data: -1)
+    allow(Gitlab::UsageData).to receive_messages(count: -1, distinct_count: -1, estimate_batch_distinct_count: -1, sum: -1)
+    allow(Gitlab::UsageData).to receive(:alt_usage_data).and_wrap_original do |m, *args, **kwargs|
+      kwargs[:fallback] || Gitlab::Utils::UsageData::FALLBACK
+    end
     allow(Gitlab::Geo).to receive(:enabled?).and_return(true)
     stub_licensed_features(requirements: true)
     stub_prometheus_queries