feat(metrics): Preliminary work for metrics aggregation

[mitsuhiko]

This PR is not for merging. It's a draft implementation of metrics aggregation that was not for merging. The actual work is in #618

[codecov]

Codecov Report

Merging #621 (dca1b6c) into master (522c901) will decrease coverage by 2.87%.
Report is 1 commits behind head on master.
The diff coverage is 23.19%.

❗ Current head dca1b6c differs from pull request most recent head 3ff5e6e. Consider uploading reports for the commit 3ff5e6e to get more accurate results

@@            Coverage Diff             @@
##           master     #621      +/-   ##
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- Coverage   73.11%   70.24%   -2.87%     
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  Files          58       60       +2     
  Lines        6576     6977     +401     
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+ Hits         4808     4901      +93     
- Misses       1768     2076     +308