stop the timer when the session's run loop returns

[marten-seemann]

Fixes #2515.

[codecov-io]

Codecov Report

Merging #2516 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2516   +/-   ##
=======================================
  Coverage   86.13%   86.14%           
=======================================
  Files         122      122           
  Lines        9533     9536    +3     
=======================================
+ Hits         8211     8214    +3     
  Misses        984      984           
  Partials      338      338           

Impacted Files	Coverage Δ
internal/utils/timer.go	100.00% <100.00%> (ø)
session.go	75.34% <100.00%> (+0.03%)	⬆️

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