fix(driver): a CPU sample the driver never took is absent, not zero

CDP exposes no per-page CPU, so chrome's Metrics filled CPUPercent with a zero,
which is the same answer an idle app gives. Making the field optional is the
same fix as the iOS reads: absence is the only honest way to say nothing looked.

internal/driver/driver.go

@@ -183,7 +183,10 @@ type Health struct {
 }
 
 type Metrics struct {
-	CPUPercent       float64
+	// CPUPercent is nil on a driver that samples memory but not CPU, which
+	// Chrome does: zero is a reading an idle app can produce, so a driver that
+	// never looked has to answer with nothing rather than with that.
+	CPUPercent       *float64
 	HeapBytes        int64
 	TotalMemoryBytes int64
 }

internal/driver/sidecar/client.go

@@ -300,8 +300,9 @@ func (c *Client) Metrics(ctx context.Context, bundleID string) (driver.Metrics,
 	if err != nil {
 		return driver.Metrics{}, err
 	}
+	cpuPercent := response.GetCpuPercent()
 	return driver.Metrics{
-		CPUPercent:       response.GetCpuPercent(),
+		CPUPercent:       &cpuPercent,
 		HeapBytes:        response.GetHeapBytes(),
 		TotalMemoryBytes: response.GetTotalMemoryBytes(),
 	}, nil

internal/driver/sidecar/client_test.go

@@ -651,7 +651,8 @@ func TestClient_MetricsMapsResponseFields(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if got.CPUPercent != 12.5 || got.HeapBytes != 100 || got.TotalMemoryBytes != 200 {
+	if got.CPUPercent == nil || *got.CPUPercent != 12.5 ||
+		got.HeapBytes != 100 || got.TotalMemoryBytes != 200 {
 		t.Errorf("metrics mapping wrong: %+v", got)
 	}
 	if len(state.fake.metricsBundles) != 1 || state.fake.metricsBundles[0] != "com.example" {