Allow restricting phases during which a `Metric` runs

[lorenzoh]

This would allow running expensive metrics only during validation phases.

Currently, you can pass functions or a Metric to the Metrics callback:

cb = Metrics(
    accuracy,
    Metric(Flux.mse),
    Metric(expensivemetricfn)
)

This feature would add a phase argument to Metric:

cb = Metrics(
    accuracy,
    Metric(Flux.mse),
    Metric(expensivemetricfn, phase = AbstractValidationPhase)
)

In case expensivemetricfn takes a long time to evaluate, this saves time during training while giving important information during the (usually much shorter) validation phases.