How to compose two metrics?

[celsofranssa]

Currently, I am computing two metrics at the validation_epoch_end, as shown in the code snippet below.

class LTModel(pl.LightningModule):

    def __init__(self, hparams):
        super(LTModel, self).__init__()
        self.entity_cls_head = # entity classifier 
        self.relation_cls_head = # relation classifier

        # metrics
        self.entity_metrics = self._get_metrics(prefix="entity_")
        self.relation_metrics = self._get_metrics(prefix="relation_")

        # loss
        self.loss = ...

    def _get_metrics(self, prefix):
        pass
        return MetricCollection(
             metrics={
                 "Wei-F1": F1(num_classes=self.hparams.num_entities, average="weighted"),
                 ...
             },
             prefix=prefix)

However, I would like to aggregate these two metrics to form a single one.
Then, is there an example I can use to achieve this goal?