Add Metrics::Timing.duration_(start,end)

[solnic]

This is just a quick follow-up after merging #2471

• Adds a helper for measuring duration
• Adds expectations for duration to check ins spec (this wouldn't pass before Improve the accuracy of duration calculations in cron jobs monitoring #2471)

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.20%. Comparing base (bb9ce72) to head (fda0638).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2501      +/-   ##
==========================================
+ Coverage   98.17%   98.20%   +0.02%     
==========================================
  Files         128      128              
  Lines        4833     4837       +4     
==========================================
+ Hits         4745     4750       +5     
+ Misses         88       87       -1     

Components	Coverage Δ
sentry-ruby	98.57% <100.00%> (+<0.01%)	⬆️
sentry-rails	97.07% <ø> (+0.15%)	⬆️
sentry-sidekiq	97.42% <ø> (ø)
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)
sentry-ruby/lib/sentry/metrics/timing.rb	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes