Add integration test to test scope is flushed correctly for shortly l…

…ived programs (#92)

There were some reporting metrics that haven't been flushed on program shutdown, this is likely due to program lifecycle however since this test is unable to reproduce the problem:
```
$ go test -v -run TestIntegrationProcessFlushOnExit -count 100
=== RUN   TestIntegrationProcessFlushOnExit
Test 0 emitted:
[Metric({Name:my-gauge MetricValue:MetricValue({Count:<nil> Gauge:<nil> Timer:TimerValue({I64Value:0xc000098980 DValue:<nil>})}) Timestamp:0xc000098990 Tags:map[]}) Me
tric({Name:my-counter MetricValue:MetricValue({Count:CountValue({I64Value:0xc0000989b8}) Gauge:<nil> Timer:<nil>}) Timestamp:0xc0000989c8 Tags:map[]}) Metric({Name:my-
gauge MetricValue:MetricValue({Count:<nil> Gauge:GaugeValue({I64Value:<nil> DValue:0xc0000989f0}) Timer:<nil>}) Timestamp:0xc000098a00 Tags:map[]})]

...

Test 4 emitted:
[Metric({Name:my-gauge MetricValue:MetricValue({Count:<nil> Gauge:<nil> Timer:TimerValue({I64Value:0xc000099330 DValue:<nil>})}) Timestamp:0xc000099340 Tags:map[]}) Me
tric({Name:my-counter MetricValue:MetricValue({Count:CountValue({I64Value:0xc000099368}) Gauge:<nil> Timer:<nil>}) Timestamp:0xc000099378 Tags:map[]}) Metric({Name:my-
gauge MetricValue:MetricValue({Count:<nil> Gauge:GaugeValue({I64Value:<nil> DValue:0xc0000993a0}) Timer:<nil>}) Timestamp:0xc0000993b0 Tags:map[]})]
--- PASS: TestIntegrationProcessFlushOnExit (4.33s)

...
```

Repeating the tests 100 iterations all pass.

m3/reporter_integration_test.go

@@ -0,0 +1,113 @@
+// Copyright (c) 2019 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package m3
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+var (
+	mainFileFmt = `
+package main
+
+import (
+	"time"
+
+	"github.com/uber-go/tally"
+	"github.com/uber-go/tally/m3"
+)
+
+func main() {
+	r, err := m3.NewReporter(m3.Options{
+		HostPorts: []string{"%s"},
+		Service:   "test-service",
+		Env:       "test",
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	scope, closer := tally.NewRootScope(tally.ScopeOptions{
+		CachedReporter: r,
+	}, 5 * time.Second)
+	defer closer.Close()
+
+	scope.Counter("my-counter").Inc(42)
+	scope.Gauge("my-gauge").Update(123)
+	scope.Timer("my-timer").Record(456 * time.Millisecond)
+}
+	`
+)
+
+// TestIntegrationProcessFlushOnExit tests whether data is correctly flushed
+// when the scope is closed for shortly lived programs
+func TestIntegrationProcessFlushOnExit(t *testing.T) {
+	for i := 0; i < 5; i++ {
+		testProcessFlushOnExit(t, i)
+	}
+}
+
+func testProcessFlushOnExit(t *testing.T, i int) {
+	dir, err := ioutil.TempDir("", "foo")
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+
+	var wg sync.WaitGroup
+	server := newFakeM3Server(t, &wg, true, Compact)
+	go server.Serve()
+	defer server.Close()
+
+	mainFile := path.Join(dir, "main.go")
+	mainFileContents := fmt.Sprintf(mainFileFmt, server.Addr)
+
+	fileErr := ioutil.WriteFile(mainFile, []byte(mainFileContents), 0666)
+	require.NoError(t, fileErr)
+
+	binary := path.Join(dir, "m3testemit")
+
+	// build
+	cmd := exec.Command("go", "build", "-o", binary, mainFile)
+	cmd.Dir = dir
+	output, err := cmd.CombinedOutput()
+	require.NoError(t, err, fmt.Sprintf("output:\n\n%s", output))
+
+	// run, do not sleep at end of the main program as per
+	// main program source code
+	wg.Add(1)
+	require.NoError(t, exec.Command(binary).Run())
+
+	// Wait for fake M3 server to receive the batch
+	wg.Wait()
+
+	require.Equal(t, 1, len(server.Service.getBatches()))
+	require.NotNil(t, server.Service.getBatches()[0])
+	require.Equal(t, 3, len(server.Service.getBatches()[0].GetMetrics()))
+	metrics := server.Service.getBatches()[0].GetMetrics()
+	fmt.Printf("Test %d emitted:\n%v\n", i, metrics)
+}