table: show nested metrics as separate columns

[dberenbaum]

Before:

train	epoch	step
{'loss': 0.0609191469848156}	4	999

After:

train.loss	epoch	step
0.0579433	4	999

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.06 🎉

Comparison is base (8aab523) 94.49% compared to head (8fd0d73) 94.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   94.49%   94.55%   +0.06%     
==========================================
  Files          19       19              
  Lines         726      734       +8     
  Branches       76      114      +38     
==========================================
+ Hits          686      694       +8     
  Misses         27       27              
  Partials       13       13              

Impacted Files	Coverage Δ
src/dvc_render/utils.py	100.00% <100.00%> (ø)
tests/test_table.py	100.00% <100.00%> (ø)

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[daavoo]

@dberenbaum you need to fix mypy