MLFlowLogger: log system metrics

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### Description & Motivation

I am using the `MLFlowLogger` to keep track of my experiments. To my knowledge, there is no possibility to enable the logging of system metrics: https://mlflow.org/docs/latest/system-metrics/index.html
To me, it seems that both `mlflow.enable_system_metrics_logging()` and the environment variable `MLFLOW_ENABLE_SYSTEM_METRICS_LOGGING` are ignored by `MLFlowLogger`.

Would it be possible to add the option? I think the most sensible would be to add an argument to `MLFlowLogger` mirroring the behavior of `mlflow.start_run(log_system_metrics=True)`.

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As I am already asking, would it also be possible to rename/customize the checkpoints of `log_model`? The names contain the epoch as number without leading zeros, such that they are incorrectly sorted in the MLflow interface. I would prefer to have some leading zeros such that lexicographical ordering corresponds to the ordering of the epochs.

### Pitch

_No response_

### Alternatives

_No response_

### Additional context

_No response_

cc @lantiga @borda

Start at the MLFlowLogger entry point and compare its behavior with mlflow.start_run(log_system_metrics=True), as well as the MLflow system-metrics documentation linked in the issue. Determine how system-metrics logging and checkpoint naming should be exposed, then verify that both requested behaviors work through the logger.

Written by the indexing model from the issue text.