Create a batcher with the passed batch size
subKey Identification of the metrics of this batcher
minimumBatchSize Size of batches. If the number is less than 1, then 1 is used.
Batch up incoming SingleMetric messages. If the #minimumBatchSize is not yet reached, the messages are stored
locally. Otherwise the list of messages will be forwarded to the next handler.
This method will be called for each written message that can be handled
by this encoder.