Merge pull request #187 from alphagov/metrics-test

Add unit tests to `Metrics` module

spec/services/metrics_spec.rb

@@ -0,0 +1,53 @@
+RSpec.describe Metrics do
+  describe ".increment_counter" do
+    let(:counter) { double(observe: nil) }
+
+    before do
+      stub_const("Metrics::COUNTERS", { foo: counter })
+    end
+
+    it "observes an increment of 1 for the given counter with the given labels" do
+      described_class.increment_counter(:foo, bar: "baz")
+
+      expect(counter).to have_received(:observe).with(1, bar: "baz")
+    end
+
+    it "fails gracefully if the counter does not exist" do
+      expect { described_class.increment_counter(:bar) }.not_to raise_error
+    end
+
+    it "fails gracefully if the operation raises an error" do
+      allow(counter).to receive(:observe).and_raise("boom")
+
+      expect { described_class.increment_counter(:foo) }.not_to raise_error
+    end
+  end
+
+  describe ".observe_duration" do
+    let(:histogram) { double(observe: nil) }
+
+    before do
+      stub_const("Metrics::HISTOGRAMS", { foo: histogram })
+    end
+
+    it "observes the duration of the given block for the given histogram with the given labels" do
+      described_class.observe_duration(:foo, bar: "baz") { "result" }
+
+      expect(histogram).to have_received(:observe).with(kind_of(Float), bar: "baz")
+    end
+
+    it "returns the result of the given block" do
+      expect(described_class.observe_duration(:foo) { "result" }).to eq("result")
+    end
+
+    it "fails gracefully if the histogram does not exist" do
+      expect(described_class.observe_duration(:bar) { "result" }).to eq("result")
+    end
+
+    it "fails gracefully if the operation raises an error" do
+      allow(histogram).to receive(:observe).and_raise("boom")
+
+      expect(described_class.observe_duration(:bar) { "result" }).to eq("result")
+    end
+  end
+end