change aggr to last value and update test

metrics_sdk/lib/opentelemetry/sdk/metrics/instrument/gauge.rb

@@ -39,7 +39,7 @@ def record(value, attributes: {})
 
           # TODO: replace the default aggregation to LastValue
           def default_aggregation
-            OpenTelemetry::SDK::Metrics::Aggregation::Sum.new
+            OpenTelemetry::SDK::Metrics::Aggregation::LastValue.new
           end
         end
       end

metrics_sdk/test/opentelemetry/sdk/metrics/instrument/gauge_test.rb

@@ -20,7 +20,7 @@
   it 'gauge should count -2' do
     gauge.record(-2, attributes: { 'foo' => 'bar' })
     metric_exporter.pull
-    last_snapshot = metric_exporter.metric_snapshots.last
+    last_snapshot = metric_exporter.metric_snapshots
 
     _(last_snapshot[0].name).must_equal('gauge')
     _(last_snapshot[0].unit).must_equal('smidgen')
@@ -30,4 +30,29 @@
     _(last_snapshot[0].data_points[0].value).must_equal(-2)
     _(last_snapshot[0].aggregation_temporality).must_equal(:delta)
   end
+
+  it 'gauge should count 1 for last recording' do
+    gauge.record(-2, attributes: { 'foo' => 'bar' })
+    gauge.record(1, attributes: { 'foo' => 'bar' })
+    metric_exporter.pull
+    last_snapshot = metric_exporter.metric_snapshots
+
+    _(last_snapshot.size).must_equal(1)
+    _(last_snapshot[0].data_points.size).must_equal(1)
+    _(last_snapshot[0].data_points[0].value).must_equal(1)
+  end
+
+  it 'gauge should count 1 for last recording' do
+    gauge.record(-2, attributes: { 'foo' => 'bar' })
+    gauge.record(1, attributes: { 'foo' => 'bar' })
+    gauge2 = meter.create_gauge('gauge2', unit: 'smidgen', description: 'a small amount of something')
+    gauge2.record(10, attributes: {})
+
+    metric_exporter.pull
+    last_snapshot = metric_exporter.metric_snapshots
+
+    _(last_snapshot.size).must_equal(2)
+    _(last_snapshot[0].data_points[0].value).must_equal(1)
+    _(last_snapshot[1].data_points[0].value).must_equal(10)
+  end
 end