Add OpenTelemetry instrumentation (#30130)

Co-authored-by: Juliano Costa <juliano.costa@datadoghq.com>
Co-authored-by: Robb Kidd <robbkidd@honeycomb.io>

config/initializers/opentelemetry.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+# Set OTEL_* environment variables according to OTel docs:
+# https://opentelemetry.io/docs/concepts/sdk-configuration/
+
+if ENV.keys.any? { |name| name.match?(/OTEL_.*_ENDPOINT/) }
+  require 'opentelemetry/sdk'
+  require 'opentelemetry/exporter/otlp'
+
+  require 'opentelemetry/instrumentation/active_job'
+  require 'opentelemetry/instrumentation/active_model_serializers'
+  require 'opentelemetry/instrumentation/concurrent_ruby'
+  require 'opentelemetry/instrumentation/excon'
+  require 'opentelemetry/instrumentation/faraday'
+  require 'opentelemetry/instrumentation/http'
+  require 'opentelemetry/instrumentation/http_client'
+  require 'opentelemetry/instrumentation/net/http'
+  require 'opentelemetry/instrumentation/pg'
+  require 'opentelemetry/instrumentation/rack'
+  require 'opentelemetry/instrumentation/rails'
+  require 'opentelemetry/instrumentation/redis'
+  require 'opentelemetry/instrumentation/sidekiq'
+
+  OpenTelemetry::SDK.configure do |c|
+    # use_all() attempts to load ALL the auto-instrumentations
+    # currently loaded by Ruby requires.
+    #
+    # Load attempts will emit an INFO or WARN to the console
+    # about the success/failure to wire up an auto-instrumentation.
+    # "WARN -- : Instrumentation: <X> failed to install" is most
+    # likely caused by <X> not being a Ruby library loaded by
+    # the application or the instrumentation has been explicitly
+    # disabled.
+    #
+    # To disable an instrumentation, set an environment variable
+    # along this pattern:
+    #
+    # OTEL_RUBY_INSTRUMENTATION_<X>_ENABLED=false
+    #
+    # For example, PostgreSQL and Redis produce a lot of child spans
+    # in the course of this application doing its business. To turn
+    # them off, set the env vars below, but recognize that you will
+    # be missing details about what particular calls to the
+    # datastores are slow.
+    #
+    # OTEL_RUBY_INSTRUMENTATION_PG_ENABLED=false
+    # OTEL_RUBY_INSTRUMENTATION_REDIS_ENABLED=false
+
+    c.use_all({
+      'OpenTelemetry::Instrumentation::Rack' => {
+        use_rack_events: false, # instead of events, use middleware; allows for untraced_endpoints to ignore child spans
+        untraced_endpoints: ['/health'],
+      },
+    })
+
+    c.service_name =  case $PROGRAM_NAME
+                      when /puma/ then 'mastodon/web'
+                      else
+                        "mastodon/#{$PROGRAM_NAME.split('/').last}"
+                      end
+    c.service_version = Mastodon::Version.to_s
+  end
+end