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/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