Nested metrics dictionaries now can be passed to the loggers

[alexeykarnachev]

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• Did you write any new necessary tests? Only doctest updated.
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What does this PR do?

Fixes #1507
Nested metrics dicts now can be passed to the logger.

PR review

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[codecov]

Codecov Report

Merging #1582 into master will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1582   +/-   ##
======================================
  Coverage      88%     88%           
======================================
  Files          68      68           
  Lines        3954    3957    +3     
======================================
+ Hits         3496    3499    +3     
  Misses        458     458