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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def __x_google_quota_descriptor(self, metric_costs):
 """Describes the metric costs for a call. Args: metric_costs: Dict of metric definitions to the integer cost value against that metric. Returns: A dict descriptor describing the Quota limits for the endpoint. """

    return {
        'metricCosts': {
            metric: cost for (metric, cost) in metric_costs.items()
        }
    } if metric_costs else None