Specify usage of metric tracker

[SkafteNicki]

What does this PR do?

Fixes #1409
Metric tracker supports single Metric and MetricCollection. This PR adds support for other MetricWrappers.
Additionally, warnings will be raised instead of failing if multi nested structures such as:

MetricCollection(MultiOutput(MeanSquaredError()))

where the output structures of the computed result gets too complicated/specific to process further.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1608 (bce8dc8) into master (78e9571) will decrease coverage by 50%.
The diff coverage is 95%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1608     +/-   ##
========================================
- Coverage      88%     39%    -50%     
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  Files         226     226             
  Lines       12029   12039     +10     
========================================
- Hits        10632    4668   -5964     
- Misses       1397    7371   +5974