Add log metric

[thinkall]

Why are these changes needed?

To log metric as discussed in the discord message

Hi <@1022910063620390932> quick question. I noticed that when I set a metric to optimize for, flaml doesn't log that metric to mlflow (when I start an mlflow run I'm the flaml context). It seems like flaml always logs "best_validation_loss" - I think it's 1-metric (when the metric is "bigger=better").

Is there a way to get flaml to log the optimization metric with the correct name?

Related issue number

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[qingyun-wu]

Looks good to me!