docs/example: add example on how to add opentelemetry headers/support

this must be added before starting the unleash client.
(in a rails initializer for example).

.rubocop.yml

@@ -127,6 +127,8 @@ Layout/BeginEndAlignment:
   Enabled: true
 Layout/EmptyLinesAroundAttributeAccessor:
   Enabled: true
+Layout/FirstHashElementIndentation:
+  EnforcedStyle: consistent
 Layout/SpaceAroundMethodCallOperator:
   Enabled: true
 Layout/MultilineMethodCallIndentation:

examples/extending_unleash_with_opentelemetry.rb

@@ -0,0 +1,63 @@
+# example on how to extend the unleash client with opentelemetry by monkey patching it.
+# to be added before initializing the client.
+# in rails it could be added, for example, at:
+# config/initializers/unleash.rb
+
+require 'opentelemetry-api'
+require 'unleash'
+
+module UnleashExtensions
+  module OpenTelemetry
+    TRACER = ::OpenTelemetry.tracer_provider.tracer('Unleash-Client', Unleash::VERSION)
+
+    module Client
+      def initialize(*opts)
+        UnleashExtensions::OpenTelemetry::TRACER.in_span("#{self.class.name}##{__method__}") do |_span|
+          super(*opts)
+        end
+      end
+
+      def is_enabled?(feature, *args)
+        UnleashExtensions::OpenTelemetry::TRACER.in_span("#{self.class.name}##{__method__}") do |span|
+          result = super(feature, *args)
+
+          # OpenTelemetry::SemanticConventions::Trace::FEATURE_FLAG_* is not in the `opentelemetry-semantic_conventions` gem yet
+          span.add_attributes({
+            'feature_flag.provider_name' => 'Unleash',
+            'feature_flag.key' => feature,
+            'feature_flag.variant' => result
+          })
+
+          result
+        end
+      end
+    end
+  end
+
+  module MetricsReporter
+    def post
+      UnleashExtensions::OpenTelemetry::TRACER.in_span("#{self.class.name}##{__method__}") do |_span|
+        super
+      end
+    end
+  end
+
+  module ToggleFetcher
+    def fetch
+      UnleashExtensions::OpenTelemetry::TRACER.in_span("#{self.class.name}##{__method__}") do |_span|
+        super
+      end
+    end
+
+    def save!
+      UnleashExtensions::OpenTelemetry::TRACER.in_span("#{self.class.name}##{__method__}") do |_span|
+        super
+      end
+    end
+  end
+end
+
+# MonkeyPatch here:
+::Unleash::Client.prepend UnleashExtensions::OpenTelemetry::Client
+::Unleash::MetricsReporter.prepend UnleashExtensions::OpenTelemetry::MetricsReporter
+::Unleash::ToggleFetcher.prepend UnleashExtensions::OpenTelemetry::ToggleFetcher