Add deprecation warning for Ruby 2.1 support

This PR, modelled on #1441, adds a deprecation warning for Ruby 2.1
users of ddtrace.

Ruby 2.1 has not received security updates since the
[1st of April, 2017](https://www.ruby-lang.org/en/news/2017/04/01/support-of-ruby-2-1-has-ended/)
and we, like the core team, highly recommend upgrading to a later
version.

Support for Ruby 2.1 will be dropped in a future release, probably
`0.54.0`.

Users are welcome to continue to use `< 1.0.0` for their Ruby 2.1
deployments going forward.

docs/GettingStarted.md

@@ -107,7 +107,7 @@ To contribute, check out the [contribution guidelines][contribution docs] and [d
 |       |                            | 2.4     | Full                                 | Latest              |
 |       |                            | 2.3     | Full                                 | Latest              |
 |       |                            | 2.2     | Full                                 | Latest              |
-|       |                            | 2.1     | Full                                 | Latest              |
+|       |                            | 2.1     | Deprecated                           | < 1.0.0             |
 |       |                            | 2.0     | EOL since June 7th, 2021             | < 0.50.0            |
 |       |                            | 1.9.3   | EOL since August 6th, 2020           | < 0.27.0            |
 |       |                            | 1.9.1   | EOL since August 6th, 2020           | < 0.27.0            |

lib/ddtrace/configuration.rb

@@ -3,6 +3,7 @@
 require 'ddtrace/configuration/pin_setup'
 require 'ddtrace/configuration/settings'
 require 'ddtrace/configuration/components'
+require 'ddtrace/utils/only_once'
 
 module Datadog
   # Configuration provides a unique access point for configurations
@@ -44,6 +45,8 @@ def configuration
     end
 
     def configure(target = configuration, opts = {})
+      ruby_version_deprecation_warning
+
       if target.is_a?(Settings)
         yield(target) if block_given?
 
@@ -191,5 +194,20 @@ def handle_interrupt_shutdown!
 
       nil
     end
+
+    RUBY_VERSION_DEPRECATION_ONLY_ONCE = Datadog::Utils::OnlyOnce.new
+    private_constant :RUBY_VERSION_DEPRECATION_ONLY_ONCE
+
+    def ruby_version_deprecation_warning
+      return unless Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.2')
+
+      RUBY_VERSION_DEPRECATION_ONLY_ONCE.run do
+        Datadog.logger.warn(
+          "Support for Ruby versions < 2.2 in dd-trace-rb is DEPRECATED.\n" \
+          "Last version to support Ruby < 2.2 will be 0.54.x, which will only receive critical bugfixes.\n" \
+          'Support for Ruby versions < 2.2 will be REMOVED in version 1.0.0.'
+        )
+      end
+    end
   end
 end

lib/ddtrace/version.rb

@@ -8,6 +8,7 @@ module VERSION
 
     STRING = [MAJOR, MINOR, PATCH, PRE].compact.join('.')
 
+    # Support for Ruby < 2.2 is being phased out and will be dropped in the near future.
     MINIMUM_RUBY_VERSION = '2.1.0'.freeze
 
     # Ruby 3.2 is not supported: Ruby 3.x support as implemented using *args

spec/ddtrace/configuration_spec.rb

@@ -373,6 +373,34 @@
           end
         end
       end
+
+      context 'deprecation warning' do
+        before { described_class.const_get('RUBY_VERSION_DEPRECATION_ONLY_ONCE').send(:reset_ran_once_state_for_tests) }
+
+        after { described_class.const_get('RUBY_VERSION_DEPRECATION_ONLY_ONCE').send(:reset_ran_once_state_for_tests) }
+
+        context 'with a deprecated Ruby version' do
+          before { skip 'Spec only runs on Ruby < 2.2' unless Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.2') }
+
+          it 'emits deprecation warning once' do
+            expect(Datadog.logger).to receive(:warn)
+              .with(/Support for Ruby versions < 2\.2 in dd-trace-rb is DEPRECATED/).once
+
+            test_class.configure
+            test_class.configure
+          end
+        end
+
+        context 'with a supported Ruby version' do
+          before { skip 'Spec only runs on Ruby >= 2.2' if Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.2') }
+
+          it 'emits no warnings' do
+            expect(Datadog.logger).to_not receive(:warn)
+
+            configure
+          end
+        end
+      end
     end
 
     describe '#health_metrics' do