simplify the singleflight impl

[uvletter]

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 56.05%. Comparing base (67ba75f) to head (ec9534f). Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4396      +/-   ##
==========================================
+ Coverage   56.03%   56.05%   +0.02%     
==========================================
  Files         154      154              
  Lines       40072    40065       -7     
==========================================
+ Hits        22453    22460       +7     
+ Misses      15106    15097       -9     
+ Partials     2513     2508       -5     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.