Logging callback metric with `on_epoch=False` makes it invisible to ModelCheckpoint
🐛 Bug
Logging a metric in a callback with on_epoch=False and then using it as monitor= in ModelCheckpoint causes an error.
Please reproduce using the BoringModel
https://colab.research.google.com/drive/1gruVIZALiS91B_nM6LxDSILekXejeZ5Y#scrollTo=i7XbLCXGkll9
To Reproduce
Use following BoringModel and post here

Expected behavior
the metric should be usable as a monitor regardless of whether it is reduced on epoch.
Environment
Note: Bugs with code are solved faster ! Colab Notebook should be made public !

CUDA:

GPU:

Tesla T4


available:         True
version:           10.1


Packages:

numpy:             1.19.4
pyTorch_debug:     True
pyTorch_version:   1.7.0+cu101
pytorch-lightning: 1.1.2
tqdm:              4.41.1


System:

OS:                Linux
architecture:

64bit



processor:         x86_64
python:            3.6.9
version:           #1 SMP Thu Jul 23 08:00:38 PDT 2020



Additional context