Use slices instead of linked lists for Metric

Also:

* Remove Gob encoder/decoder
* Stop using custom encoders/decoders for Timestamps (both ugorji and the Golang JSON codecs use nanosecond precision).
* Use idiomatic way to check for existence in metric.LastSample()

probe/host/reporter_test.go

@@ -88,10 +88,10 @@ func TestReporter(t *testing.T) {
 
 	// Should have metrics
 	for key, want := range metrics {
-		wantSample := want.LastSample()
+		wantSample, _ := want.LastSample()
 		if metric, ok := node.Metrics[key]; !ok {
 			t.Errorf("Expected %s metric, but not found", key)
-		} else if sample := metric.LastSample(); sample == nil {
+		} else if sample, ok := metric.LastSample(); !ok {
 			t.Errorf("Expected %s metric to have a sample, but there were none", key)
 		} else if sample.Value != wantSample.Value {
 			t.Errorf("Expected %s metric sample %f, got %f", key, wantSample, sample.Value)

probe/probe_internal_test.go

@@ -69,9 +69,6 @@ func (m mockPublisher) Stop() {
 }
 
 func TestProbe(t *testing.T) {
-	// marshalling->unmarshaling is not idempotent due to `json:"omitempty"`
-	// tags, transforming empty slices into nils. So, we make DeepEqual
-	// happy by setting empty `json:"omitempty"` entries to nil
 	const probeID = "probeid"
 	now := time.Now()
 	mtime.NowForce(now)
@@ -79,8 +76,15 @@ func TestProbe(t *testing.T) {
 
 	want := report.MakeReport()
 	node := report.MakeNodeWith("a", map[string]string{"b": "c"})
-	node.Metrics = nil // omitempty
-	// omitempty
+
+	// DeepEqual doesn't handle the location of timestamps correctly
+	// See https://github.com/golang/go/issues/10089
+	node.Controls.Timestamp = time.Now()
+
+	// marshalling->unmarshaling is not idempotent due to `json:"omitempty"`
+	// tags, transforming empty slices into nils. So, we make DeepEqual
+	// happy by setting empty `json:"omitempty"` entries to nil
+	node.Metrics = nil
 	want.Endpoint.Controls = nil
 	want.Process.Controls = nil
 	want.Container.Controls = nil

probe/process/reporter_test.go

@@ -62,7 +62,7 @@ func TestReporter(t *testing.T) {
 	}
 	if memoryUsage, ok := node.Metrics[process.MemoryUsage]; !ok {
 		t.Errorf("Expected memory usage metric, but not found")
-	} else if sample := memoryUsage.LastSample(); sample == nil {
+	} else if sample, ok := memoryUsage.LastSample(); !ok {
 		t.Errorf("Expected memory usage metric to have a sample, but there were none")
 	} else if sample.Value != 0. {
 		t.Errorf("Expected memory usage metric sample %f, got %f", 0., sample.Value)