Change default aggregation method in metric

[Ama16]

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• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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Related Issue

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[github-actions]

🚀 Deployed on https://deploy-preview-684--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #684 (f739c4f) into master (4c04584) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #684   +/-   ##
=======================================
  Coverage   83.82%   83.83%           
=======================================
  Files         119      119           
  Lines        6400     6401    +1     
=======================================
+ Hits         5365     5366    +1     
  Misses       1035     1035           

Impacted Files	Coverage Δ
etna/metrics/base.py	96.87% <100.00%> (+0.04%)	⬆️

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