Add a pull controller, use it for Prometheus (#751)

* Checkpoint new pull controller

* Tests pass

* Fix example

* Example fix

* Add a test

* Comment

* address MrAlias's feedback

sdk/metric/controller/pull/pull_test.go

@@ -0,0 +1,112 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package pull_test
+
+import (
+	"context"
+	"runtime"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"go.opentelemetry.io/otel/api/kv"
+	"go.opentelemetry.io/otel/api/label"
+	"go.opentelemetry.io/otel/api/metric"
+	"go.opentelemetry.io/otel/sdk/metric/controller/pull"
+	controllerTest "go.opentelemetry.io/otel/sdk/metric/controller/test"
+	"go.opentelemetry.io/otel/sdk/metric/integrator/test"
+	selector "go.opentelemetry.io/otel/sdk/metric/selector/simple"
+)
+
+func TestPullNoCache(t *testing.T) {
+	puller := pull.New(
+		selector.NewWithExactDistribution(),
+		pull.WithCachePeriod(0),
+		pull.WithStateful(true),
+	)
+
+	ctx := context.Background()
+	meter := puller.Provider().Meter("nocache")
+	counter := metric.Must(meter).NewInt64Counter("counter")
+
+	counter.Add(ctx, 10, kv.String("A", "B"))
+
+	puller.Collect(ctx)
+	records := test.NewOutput(label.DefaultEncoder())
+	_ = puller.ForEach(records.AddTo)
+
+	require.EqualValues(t, map[string]float64{
+		"counter/A=B/": 10,
+	}, records.Map)
+
+	counter.Add(ctx, 10, kv.String("A", "B"))
+
+	puller.Collect(ctx)
+	records = test.NewOutput(label.DefaultEncoder())
+	_ = puller.ForEach(records.AddTo)
+
+	require.EqualValues(t, map[string]float64{
+		"counter/A=B/": 20,
+	}, records.Map)
+}
+
+func TestPullWithCache(t *testing.T) {
+	puller := pull.New(
+		selector.NewWithExactDistribution(),
+		pull.WithCachePeriod(time.Second),
+		pull.WithStateful(true),
+	)
+	mock := controllerTest.NewMockClock()
+	puller.SetClock(mock)
+
+	ctx := context.Background()
+	meter := puller.Provider().Meter("nocache")
+	counter := metric.Must(meter).NewInt64Counter("counter")
+
+	counter.Add(ctx, 10, kv.String("A", "B"))
+
+	puller.Collect(ctx)
+	records := test.NewOutput(label.DefaultEncoder())
+	_ = puller.ForEach(records.AddTo)
+
+	require.EqualValues(t, map[string]float64{
+		"counter/A=B/": 10,
+	}, records.Map)
+
+	counter.Add(ctx, 10, kv.String("A", "B"))
+
+	// Cached value!
+	puller.Collect(ctx)
+	records = test.NewOutput(label.DefaultEncoder())
+	_ = puller.ForEach(records.AddTo)
+
+	require.EqualValues(t, map[string]float64{
+		"counter/A=B/": 10,
+	}, records.Map)
+
+	mock.Add(time.Second)
+	runtime.Gosched()
+
+	// Re-computed value!
+	puller.Collect(ctx)
+	records = test.NewOutput(label.DefaultEncoder())
+	_ = puller.ForEach(records.AddTo)
+
+	require.EqualValues(t, map[string]float64{
+		"counter/A=B/": 20,
+	}, records.Map)
+
+}