update getGpuUsage() to only refresh metrics for one gpu

[smedegaard]

WorkerThread.getGpuUsage() updates the metrics of all available GPUs. That's redundant since one worker thread is assigned to one GPU.

Pass the array list holding the relevnat GPU id.

Blocked by #21

    public String getGpuUsage() {
        StringBuffer gpuUsage = new StringBuffer();
        if (gpuId >= 0) {
            try {
                configManager.systemInfo.updateAcceleratorMetrics();  // <--- pass [gpuid] here
                Accelerator accelerator =
                        this.configManager.systemInfo.getAccelerators().get(gpuId);
                return accelerator.utilizationToString();
            } catch (Exception e) {
                gpuUsage.append("failed to obtained gpu usage");
                logger.error("Exception Raised : " + e.toString());
            }
        } else {
            gpuUsage.append("N/A");
        }

        return gpuUsage.toString();
    }