experimental/stats: re-add type aliases for migration

[dfawley]

#7874 removed symbols without leaving behind type aliases, which we should not do. This change re-adds the things removed and should be kept for several releases.

(No release notes since it will be in a patch release.)

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.02%. Comparing base (38a8b9a) to head (faf18cd).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
experimental/stats/metrics.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7929      +/-   ##
==========================================
- Coverage   82.10%   82.02%   -0.09%     
==========================================
  Files         378      379       +1     
  Lines       38258    38260       +2     
==========================================
- Hits        31412    31382      -30     
- Misses       5547     5566      +19     
- Partials     1299     1312      +13     

Files with missing lines	Coverage Δ
experimental/stats/metrics.go	0.00% <0.00%> (ø)

... and 24 files with indirect coverage changes

[tritone]

LGTM, thanks for the fix @dfawley