Azure Serverless extension custom metrics bug

[rodrigoazs]

In the custom metrics condition inside configure method, it should be if "metrics_sets" instead of if "metric_sets".

https://github.com/elastic/apm-agent-python/blob/d4570ce366fd7ad4c5cba782f13e5e55cb71b175/elasticapm/contrib/serverless/azure.py#L99C1-L101C44