Prometheus Metric type missing in the serialized JSON to Kafka

[ssmails]

The prometheus metric type could be included in the searlized JSON sent to Kafka. Having the metric type populated, would make it easier to work with the consumers of Kafka Eg: https://docs.confluent.io/5.5.3/connect/kafka-connect-prometheus-metrics/index.html --> This assumes the metric type is included in the json added under the kafka topic.

https://github.com/Telefonica/prometheus-kafka-adapter/blob/master/serializers.go#L58