Harden Prometheus client wrapper

changelogs/unreleased/aa-harden-with-prometheus-callback.yml

+---
+title: Harden Prometheus client usage data wrapper
+merge_request: 56210
+author:
+type: other

lib/gitlab/utils/usage_data.rb

@@ -113,11 +113,13 @@ def redis_usage_data(counter = nil, &block)
         end
       end
-      def with_prometheus_client(fallback: nil, verify: true)
+      def with_prometheus_client(fallback: {}, verify: true)
         client = prometheus_client(verify: verify)
         return fallback unless client
         yield client
+      rescue
+        fallback
       end
       def measure_duration

spec/lib/gitlab/utils/usage_data_spec.rb

@@ -246,6 +246,13 @@
   end
   describe '#with_prometheus_client' do
+    it 'returns fallback with for an exception in yield block' do
+      allow(described_class).to receive(:prometheus_client).and_return(Gitlab::PrometheusClient.new('http://localhost:9090'))
+      result = described_class.with_prometheus_client(fallback: -42) { |client| raise StandardError }
+
+      expect(result).to be(-42)
+    end
+
     shared_examples 'query data from Prometheus' do
       it 'yields a client instance and returns the block result' do
         result = described_class.with_prometheus_client { |client| client }
@@ -255,10 +262,10 @@
     end
     shared_examples 'does not query data from Prometheus' do
-      it 'returns nil by default' do
+      it 'returns {} by default' do
         result = described_class.with_prometheus_client { |client| client }
-        expect(result).to be_nil
+        expect(result).to eq({})
       end
       it 'returns fallback if provided' do