def update_available(self, name):
        """Updates the "available" metric for the given quota."""
        quota = self.usages.get(name, {}).get('quota', float('inf'))
        available = quota - self.usages[name]['used']
        if available < 0:
            available = 0
        self.usages[name]['available'] = available
Updates the "available" metric for the given quota.