Metrics

[BaruchG]

What does this PR do?

Part of Issue #839 and updates pl_bolts.metrics.aggregation.* with tests for all functions.

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• Did you write any new necessary tests? [not needed for typos/docs]
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[otaj]

Hi, @BaruchG, should I review the PR (or do you need some help with it)? Or is the draft status correct? ⚡

[BaruchG]

Hey @otaj - I was confused by what's happening with the failing tests. I've reached out over slack to discuss further. Thanks!

[BaruchG]

Ok, got this fixed. Should be ready for review.