Metric aggragation is broken for LoggerCollection
🐛 Bug

After changes in #1278 it is now not possible to log testing metrics after traning while using several loggers.
To Reproduce
Say we want to run a MINST example and also want to add a change - log testing metrics after training. For that we define a Callback
class TestCallback(Callback):
    def on_train_end(self, trainer, pl_module):
        # note that it would crash if you don't pass the `pl_module`
        trainer.test(pl_module)

and pass it to trainer callbacks argument.
We would also like to use several loggers to track all metrics, say MLFlowLogger and TensorBoardLogger. For this we create instances of these loggers and pass them into Trainer in a list.
Expected behavior
Testing metrics should be logged - but they don't as there's no final aggregation when our logger is a LoggerCollection
Additional context
In my opinion, the logic in agg_and_log_metrics  and _finalize_agg_metrics is hard to follow, so I'd be happy if user could choose plain old log_metrics which worked nicely.