Refactor collection_jitter and flush_jitter

use a common function between collection_jitter and flush_jitter. which
creates the same behavior between the two options.

going forward, both jitters will be random sleeps that get re-evaluated
at runtime for every interval (previously only collection_jitter did
this)

also fixes behavior so that both jitters will exit in the event of a
process exit.

closes #1296

internal/internal_test.go

@@ -137,3 +137,28 @@ func TestCompileFilter(t *testing.T) {
 	assert.True(t, f.Match("mem"))
 	assert.True(t, f.Match("network"))
 }
+
+func TestRandomSleep(t *testing.T) {
+	// test that zero max returns immediately
+	s := time.Now()
+	RandomSleep(time.Duration(0), make(chan struct{}))
+	elapsed := time.Since(s)
+	assert.True(t, elapsed < time.Millisecond)
+
+	// test that max sleep is respected
+	s = time.Now()
+	RandomSleep(time.Millisecond*50, make(chan struct{}))
+	elapsed = time.Since(s)
+	assert.True(t, elapsed < time.Millisecond*50)
+
+	// test that shutdown is respected
+	s = time.Now()
+	shutdown := make(chan struct{})
+	go func() {
+		time.Sleep(time.Millisecond * 100)
+		close(shutdown)
+	}()
+	RandomSleep(time.Second, shutdown)
+	elapsed = time.Since(s)
+	assert.True(t, elapsed < time.Millisecond*150)
+}