What's the Drupal equivalent of Sentry?
You can use New Relic.


  Database performance
  
  Viewing slow SQL queries is a breeze. Most Drupal sites rely heavily
  on Views and CCK which can generate non-optimal SQL queries. New Relic
  allows you to quickly pinpoint these types of queries.
  
  Modules Monitoring
  
  You can identify which modules are slowest by call count or by slowest
  call time. This may help you identify which modules may be running
  rogue within your application.
  
  Apdex
  
  New Relic uses Apdex scoring to help you evaluate how well your Drupal
  application is performing for your end-users according to an
  industry-standard measurement. This helps you provide better data as
  well as have a clearer understanding of what percentage of your users
  are frustrated or satisfied. Apdex scoring also provides more specific
  context than a general “the site is slow” complaint that may not even
  get to you until hours after the incident. Apdex also gives you a more
  objective way to measure your end-users’ experience with your
  performance improvements.
  
  Function Performance
  
  Using Web Transaction Traces, you can see which functions are the
  slowest in your application. You can use this to identify slow
  functions in custom modules you have written, or to track delays in
  your application caused by community module functions that do things
  like authenticate with third-party systems.
  
  Front-end performance
  
  All that wonderful flexibility within blocks, views, and panels can
  sometimes result in complex page renderings that need to be processed
  by the browsers of your end-users. New Relic’s Real User Monitoring
  (RUM) will actually gather timing information and show you which
  hotspots in DOM rendering time may be causing your page to take
  several seconds to load.