[RLlib] Add reset() method to MetricsLogger.

[sven1977]

Add reset() method to MetricsLogger.

Why are these changes needed?

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[simonsays1980]

LGTM. Great PR! This is still needed. To cover some use cases: Is it possible to pass in a list of keys to reset while keeping the rest?

[simonsays1980]

Is it possible to turn over a list of keys to reset, while keeping others? e.g. resetting all performance metrics, but keeping the counters.

[sven1977]

Ah, yes, for that, you'd use:

logger.delete(*[keys_to_delete])