const usageTracker = async (req, res, next) => {
  const startTime = Date.now();
  const apiKey = req.headers['x-api-key'];
  const endpoint = req.path;
  
  // Get user's current plan
  const user = await getUserByApiKey(apiKey);
  const plan = await getUserPlan(user.id);
  
  // Check usage limits
  const currentUsage = await getCurrentMonthUsage(user.id);
  if (currentUsage >= plan.limits.requestsPerMonth) {
    return res.status(429).json({
      error: 'Monthly usage limit exceeded',
      current: currentUsage,
      limit: plan.limits.requestsPerMonth
    });
  }
  
  // Track the request
  res.on('finish', async () => {
    const duration = Date.now() - startTime;
    const responseSize = res.get('content-length') || 0;
    
    await recordUsage({
      userId: user.id,
      endpoint,
      method: req.method,
      statusCode: res.statusCode,
      duration,
      responseSize,
      timestamp: new Date()
    });
    
    // Calculate billing
    const cost = calculateRequestCost(plan, endpoint, responseSize);
    await updateUserBilling(user.id, cost);
  });
  
  next();
};