Improve the accuracy of duration calculations in cron jobs monitoring

[krzysztofjablonski]

This pull request includes changes to the perform method in the sentry-ruby/lib/sentry/cron/monitor_check_ins.rb file to improve the accuracy of duration calculations.

• Changed the duration calculation to use floating-point precision instead of integer precision to improve accuracy. [1] [2]

Before:

After:

[solnic]

Thank you for the PR! Actually, we should probably consider adding a helper for the duration and use the approach described here: https://blog.dnsimple.com/2018/03/elapsed-time-with-ruby-the-right-way/

This should give the best results.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.19%. Comparing base (a9b3687) to head (142b243).
Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2471      +/-   ##
==========================================
+ Coverage   98.17%   98.19%   +0.02%     
==========================================
  Files         128      128              
  Lines        4825     4833       +8     
==========================================
+ Hits         4737     4746       +9     
+ Misses         88       87       -1     

Components	Coverage Δ
sentry-ruby	98.57% <100.00%> (+<0.01%)	⬆️
sentry-rails	97.07% <ø> (-0.01%)	⬇️
sentry-sidekiq	97.42% <ø> (+0.45%)	⬆️
sentry-resque	92.85% <ø> (ø)
sentry-delayed_job	95.65% <ø> (ø)
sentry-opentelemetry	99.31% <ø> (ø)

Files with missing lines	Coverage Δ
sentry-ruby/lib/sentry/cron/monitor_check_ins.rb	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes