How do I measure and improve the efficiency of Envoy?

     0,
        'latency' => [],
        'error_count' => 0
    ];

    function trackRequest($startTime) {
        global $metrics;
        $metrics['request_count']++;
        $endTime = microtime(true);
        $latency = ($endTime - $startTime) * 1000; // latency in milliseconds
        $metrics['latency'][] = $latency;
    }

    // Example request handling
    $startTime = microtime(true);
    // ... process request ...
    trackRequest($startTime);

    // Calculate average latency
    $averageLatency = array_sum($metrics['latency']) / count($metrics['latency']);
    echo "Average Latency: " . $averageLatency . " ms";
    echo "Total Requests: " . $metrics['request_count'];
    echo "Total Errors: " . $metrics['error_count'];
    ?>