Misleading docs or impl for log_metrics

[clumsy]

Bug description

The docs currently say:

            step: Step for which metrics should be logged. Default value is `self.global_step` during training or
                the total validation / test log step count during validation and testing.

But during training if someone is to do LightningModule.log("step", 42.0) it would be placed into metrics and used before we fallback to trainer.global_step a few lines below.

E.g. NeMo does log this metric unconditionally: https://github.com/NVIDIA/NeMo/blob/fae8897686d7460381b016de005b44ff27bd1092/nemo/lightning/pytorch/strategies/megatron_strategy.py#L675

Either the docs need to be fixed, or we need to only fallback to metrics for step when not training. If we do fallback - we need to ensure step is an int (convertible from float w/o loss).

Happy to contribute should we figure out which route to go.

What version are you seeing the problem on?

master

How to reproduce the bug

`LightningModule.log("step", 1.0)`, then `LightningModule.log("metric", 42.0)` and then observe `<Metric: key='metric', step=1.0, timestamp=1743030659681, value=42.0>`

Note that this makes the step a `float` vs. `int`.

Error messages and logs

# Error messages and logs here please

Environment

Current environment

#- PyTorch Lightning Version (e.g., 2.5.0):
#- PyTorch Version (e.g., 2.5):
#- Python version (e.g., 3.12):
#- OS (e.g., Linux):
#- CUDA/cuDNN version:
#- GPU models and configuration:
#- How you installed Lightning(`conda`, `pip`, source):

More info

No response

[SkafteNicki]

@clumsy thanks for raising this issue. Are you still interested in contributing?
I think the correct path forward is just to change the docstring to say something like

"""
step: Step for which metrics should be logged. If a `step` metric is logged, this value will 
    be used else will default to `self.global_step` during training or the total validation / test log 
    step count during validation and testing.
"""

[clumsy]

Sounds good, @SkafteNicki. Since #20830 was merged we only need to fix the docs. I'll send a PR shortly.