spec/metrics: add specs for upstream metrics

spec/metrics/upstream_spec.lua

@@ -0,0 +1,59 @@
+describe('upstream metrics', function()
+  describe('report', function()
+    local upstream_metrics
+    local test_counter = { inc = function() end }
+    local test_histogram = { observe = function() end }
+
+    before_each(function()
+      -- Make Prometheus return stubbed counters and histograms
+      stub(test_counter, 'inc')
+      stub(test_histogram, 'observe')
+
+      local Prometheus = require('apicast.prometheus')
+      getmetatable(Prometheus).__call = function(_, type)
+        if type == 'counter' then
+          return test_counter
+        elseif type == 'histogram' then
+          return test_histogram
+        end
+      end
+
+      package.loaded['apicast.metrics.upstream'] = nil
+      upstream_metrics = require('apicast.metrics.upstream')
+    end)
+
+    after_each(function()
+      package.loaded['apicast.prometheus'] = nil
+      require('apicast.prometheus')
+
+      package.loaded['apicast.metrics.upstream'] = nil
+      require('apicast.metrics.upstream')
+    end)
+
+    it('increases the counter of status codes', function()
+      upstream_metrics.report(200, 0.1)
+      assert.stub(test_counter.inc).was_called_with(test_counter, 1, { 200 })
+    end)
+
+    it('adds the latency to the histogram', function()
+      upstream_metrics.report(200, 0.1)
+      assert.stub(test_histogram.observe).was_called_with(test_histogram, 0.1)
+    end)
+
+    describe('when the status is nil or empty', function()
+      it('does not increase the counter of status codes', function()
+        upstream_metrics.report(nil, 0.1)
+        upstream_metrics.report('', 0.1)
+        assert.stub(test_counter.inc).was_not_called()
+      end)
+    end)
+
+    describe('when the latency is nil or empty', function()
+      it('does not add the latency to the histogram', function()
+        upstream_metrics.report(200, nil)
+        upstream_metrics.report(200, '')
+        assert.stub(test_histogram.observe).was_not_called()
+      end)
+    end)
+  end)
+end)