feat: optionally allow APM using Sentry via env variable

Gemfile

@@ -42,3 +42,8 @@ group :test do
   gem 'vcr'
   gem 'webmock'
 end
+
+group :sentry do
+  gem 'sentry-ruby'
+  gem 'stackprof'
+end

Gemfile.lock

@@ -135,13 +135,17 @@ GEM
     sanitize (6.1.3)
       crass (~> 1.0.2)
       nokogiri (>= 1.12.0)
+    sentry-ruby (5.21.0)
+      bigdecimal
+      concurrent-ruby (~> 1.0, >= 1.0.2)
     simplecov (0.22.0)
       docile (~> 1.1)
       simplecov-html (~> 0.11)
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
     ssrf_filter (1.1.2)
+    stackprof (0.2.26)
     thor (1.3.2)
     tilt (2.4.0)
     tzinfo (2.0.6)
@@ -185,8 +189,10 @@ DEPENDENCIES
   rubocop-rake
   rubocop-rspec
   rubocop-thread_safety
+  sentry-ruby
   simplecov
   ssrf_filter
+  stackprof
   tilt
   vcr
   webmock

README.md

@@ -162,6 +162,8 @@ If you're going to host a public instance, _please, please, please_:
 | `RACK_TIMEOUT_SERVICE_TIMEOUT` | default: 15                        |
 | `WEB_CONCURRENCY`              | default: 2                         |
 | `WEB_MAX_THREADS`              | default: 5                         |
+|                                |                                    |
+| `SENTRY_DSN`                   | no default.                        |
 
 ### Runtime monitoring via `GET /health_check.txt`
 
@@ -181,6 +183,10 @@ Create a monitor of type _Keyword_ with this information and make it aware of yo
 
 ![A screenshot showing the Keyword Monitor: a name, the instance's URL to /health_check.txt, and an interval.](docs/uptimerobot_monitor.jpg)
 
+### Application Performance Monitoring using Sentry
+
+When you specify `SENTRY_DSN` in your environment variables, the application will be setup to use Sentry.
+
 ## Setup for development
 
 Check out the git repository and…

config.ru

@@ -4,8 +4,31 @@ require 'rubygems'
 require 'bundler/setup'
 require 'rack-timeout'
 
-dev = ENV.fetch('RACK_ENV', nil) == 'development'
+if ENV.key?('SENTRY_DSN')
+  Bundler.require(:sentry)
+  require 'sentry-ruby'
+
+  Sentry.init do |config|
+    config.dsn = ENV.fetch('SENTRY_DSN')
+
+    # Set traces_sample_rate to 1.0 to capture 100%
+    # of transactions for tracing.
+    # We recommend adjusting this value in production.
+    config.traces_sample_rate = 1.0
+    # or
+    # config.traces_sampler = lambda do |_context|
+    #   true
+    # end
+    # Set profiles_sample_rate to profile 100%
+    # of sampled transactions.
+    # We recommend adjusting this value in production.
+    config.profiles_sample_rate = 1.0
+  end
+
+  use Sentry::Rack::CaptureExceptions
+end
 
+dev = ENV.fetch('RACK_ENV', nil) == 'development'
 requires = Dir['app/**/*.rb']
 
 if dev