- Added `Accuracy` metric now generalizes to Top-k accuracy for (multi-dimensional) multi-class inputs using the `top_k` parameter ([PL^4838](https://github.com/PyTorchLightning/pytorch-lightning/pull/4838))
- Added `Accuracy` metric now enables the computation of subset accuracy for multi-label or multi-dimensional multi-class inputs with the `subset_accuracy` parameter ([PL^4838](https://github.com/PyTorchLightning/pytorch-lightning/pull/4838))
- Added `HammingDistance` metric to compute the hamming distance (loss) ([PL^4838](https://github.com/PyTorchLightning/pytorch-lightning/pull/4838))
- Added `StatScores` metric to compute the number of true positives, false positives, true negatives and false negatives ([PL^4839](https://github.com/PyTorchLightning/pytorch-lightning/pull/4839))
- Added `.clone()` method to metrics ([PL^4318](https://github.com/PyTorchLightning/pytorch-lightning/pull/4318))
- Added `IoU` class interface ([PL^4704](https://github.com/PyTorchLightning/pytorch-lightning/pull/4704))
- The `Recall` and `Precision` metrics (and their functional counterparts `recall` and `precision`) can now be generalized to Recall@K and Precision@K with the use of `top_k` parameter ([PL^4842](https://github.com/PyTorchLightning/pytorch-lightning/pull/4842))
- Added `F1` class metric ([PL^4656](https://github.com/PyTorchLightning/pytorch-lightning/pull/4656))
- Added metrics aggregation in Horovod and fixed early stopping ([PL^3775](https://github.com/PyTorchLightning/pytorch-lightning/pull/3775))
- Added `persistent(mode)` method to metrics, to enable and disable metric states being added to `state_dict` ([PL^4482](https://github.com/PyTorchLightning/pytorch-lightning/pull/4482))
- Added unification of regression metrics ([PL^4166](https://github.com/PyTorchLightning/pytorch-lightning/pull/4166))
- Added persistent flag to `Metric.add_state` ([PL^4195](https://github.com/PyTorchLightning/pytorch-lightning/pull/4195))