Expose negate_key_metric argument in Job API.

[holgerroth]

          This is different, what metrics you pass in vs. how best model selector behaviors

for example if the metrics is "accuracy", or "auc"

 the best model selection criteria will be   metrics_new > metrics_old

if the metrics is "loss"
the best model selection criteria will be metrics_new < metrics_old

Does best model selection handle these ? how does the model know metrics criteria to use (> or <) for given metrics name ?

Originally posted by @chesterxgchen in #2563 (comment)