Emit log message for incompatible Lograge setup

Gemfile

@@ -93,3 +93,10 @@ end
 #
 # TODO: Remove this once the issue is resolved: https://github.com/ffi/ffi/issues/1107
 gem 'ffi', '~> 1.16.3', require: false
+
+gem 'rails', '~> 6.1.0'
+gem 'pg', '>= 1.1', platform: :ruby
+gem 'sprockets', '< 4'
+gem 'lograge', '~> 0.11'
+gem 'net-smtp'
+gem 'grpc'

lib/datadog/tracing/contrib/lograge/patcher.rb

@@ -20,6 +20,20 @@ def target_version
 
           # patch applies our patch
           def patch
+            # ActiveSupport::TaggedLogging is the default Rails logger since Rails 5
+            if defined?(::Rails::VERSION::MAJOR) && ::Rails::VERSION::MAJOR >= 5
+              Datadog.logger.error(
+                "Lograge and ActiveSupport::TaggedLogging (the default Rails logger) are not compatible: " \
+                  "Lograge does not account for Rails log tags, creating polluted logs and breaking log formatting. " \
+                  "Traces and Logs correlation may not work. " \
+                  "Either: 1. Disable tagged logging in your Rails configuration " \
+                  "`config.logger = ActiveSupport::Logger.new(STDOUT); " \
+                  "config.active_job.logger = ActiveSupport::Logger.new(STDOUT)` " \
+                  "or 2. Use the `semantic_logger` gem instead of `lograge`."
+              )
+            end
+
+
             ::Lograge::LogSubscribers::Base.include(Instrumentation)
           end
         end

spec/datadog/tracing/contrib/lograge/patcher_spec.rb

@@ -5,10 +5,32 @@
 
 RSpec.describe Datadog::Tracing::Contrib::Lograge::Patcher do
   describe '.patch' do
+    before { described_class.instance_variable_get(:@patch_only_once)&.send(:reset_ran_once_state_for_tests) }
+
     it 'adds Instrumentation to ancestors of LogSubscribers::Base class' do
       described_class.patch
 
       expect(Lograge::LogSubscribers::Base.ancestors).to include(Datadog::Tracing::Contrib::Lograge::Instrumentation)
     end
+
+    context 'with older Rails' do
+      it 'does not log incompatibility error' do
+        stub_const('Rails::VERSION::MAJOR', 4)
+
+        expect(Datadog.logger).to_not receive(:error)
+
+        described_class.patch
+      end
+    end
+
+    context 'with Rails with tagged logging' do
+      it 'logs an incompatibility error' do
+        stub_const('Rails::VERSION::MAJOR', 5)
+
+        expect(Datadog.logger).to receive(:error).with(/ActiveSupport::TaggedLogging/)
+
+        described_class.patch
+      end
+    end
   end
 end