Add custom fallback to GenericMetric

lib/gitlab/usage/metrics/instrumentations/generic_metric.rb

@@ -12,19 +12,25 @@ module Gitlab
           #     Gitlab::CurrentSettings.uuid
           #   end
           # end
+          FALLBACK = -1
+
           class << self
             attr_reader :metric_operation
             @metric_operation = :alt
 
+            def fallback(custom_fallback)
+              @metric_fallback = custom_fallback
+            end
+
             def value(&block)
               @metric_value = block
             end
 
-            attr_reader :metric_value
+            attr_reader :metric_value, :metric_fallback
           end
 
           def value
-            alt_usage_data do
+            alt_usage_data(fallback: fallback) do
               self.class.metric_value.call
             end
           end
@@ -34,6 +40,10 @@ module Gitlab
               self.class.metric_operation
             )
           end
+
+          def fallback
+            self.class.metric_fallback || FALLBACK
+          end
         end
       end
     end

spec/lib/gitlab/usage/metrics/instrumentations/generic_metric_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::Metrics::Instrumentations::GenericMetric do
+  context 'with custom fallback' do
+    subject do
+      Class.new(described_class) do
+        fallback(-2)
+        value { Gitlab::Database.version }
+      end.new(time_frame: 'none')
+    end
+
+    describe '#value' do
+      it 'gives the correct value' do
+        expect(subject.value).to eq(Gitlab::Database.version )
+      end
+
+      context 'when raising an exception' do
+        it 'return the custom fallback' do
+          expect(Gitlab::Database).to receive(:version).and_raise('Error')
+          expect(subject.value).to eq(-2)
+        end
+      end
+    end
+  end
+
+  context 'with default fallback' do
+    subject do
+      Class.new(described_class) do
+        value { Gitlab::Database.version }
+      end.new(time_frame: 'none')
+    end
+
+    describe '#value' do
+      it 'gives the correct value' do
+        expect(subject.value).to eq(Gitlab::Database.version )
+      end
+
+      context 'when raising an exception' do
+        it 'return the custom fallback' do
+          expect(Gitlab::Database).to receive(:version).and_raise('Error')
+          expect(subject.value).to eq(-1)
+        end
+      end
+    end
+  end
+end