Allow to disable instrumentation

lib/dalli/instrumentation.rb

@@ -61,13 +61,14 @@ class << self
       # Uses the library name 'dalli' and current Dalli::VERSION.
       #
       # @return [OpenTelemetry::Trace::Tracer, nil] the tracer or nil if OTel unavailable
-      # rubocop:disable ThreadSafety/ClassInstanceVariable
+      # rubocop:disable ThreadSafety/ClassInstanceVariable, ThreadSafety/ClassAndModuleAttributes
       def tracer
         return @tracer if defined?(@tracer)
 
         @tracer = (OpenTelemetry.tracer_provider.tracer('dalli', Dalli::VERSION) if defined?(OpenTelemetry))
       end
-      # rubocop:enable ThreadSafety/ClassInstanceVariable
+
+      attr_writer :tracer
 
       # Returns true if instrumentation is enabled (OpenTelemetry SDK is available).
       #
@@ -76,6 +77,15 @@ def enabled?
         !tracer.nil?
       end
 
+      # Disable instrumentation.
+      #
+      # @return [nil]
+      def disable!
+        @tracer = nil
+      end
+
+      # rubocop:enable ThreadSafety/ClassInstanceVariable, ThreadSafety/ClassAndModuleAttributes
+
       # Wraps a block with a span if instrumentation is enabled.
       #
       # Creates a client span with the given name and attributes merged with

test/test_instrumentation.rb

@@ -91,6 +91,18 @@ def clear_tracer_cache
     end
   end
 
+  describe '.disable!' do
+    it 'causes .enabled? to return false' do
+      Dalli::Instrumentation.tracer = MockOpenTelemetry::MockTracer.new
+
+      assert_predicate Dalli::Instrumentation, :enabled?
+
+      Dalli::Instrumentation.disable!
+
+      refute_predicate Dalli::Instrumentation, :enabled?
+    end
+  end
+
   describe '.trace without OpenTelemetry' do
     it 'yields the block and returns its result when tracing is disabled' do
       clear_tracer_cache