Adds on_ending callback to allow processors to mutate spans before End operation

sdk/lib/opentelemetry/sdk/trace/span.rb

@@ -271,6 +271,9 @@ def finish(end_timestamp: nil)
               return self
             end
             @end_timestamp = relative_timestamp(end_timestamp)
+            @span_processors.each do |processor|
+              processor.on_finishing(self) if processor.respond_to?(:on_finishing)
+            end
             @attributes = validated_attributes(@attributes).freeze
             @events.freeze
             @links.freeze

sdk/lib/opentelemetry/sdk/trace/span_processor.rb

@@ -23,6 +23,24 @@ class SpanProcessor
         #  started span.
         def on_start(span, parent_context); end
 
+        # The on_finishing method is an experimental feature and may have breaking changes.
+        # The OpenTelemetry specification defines it as "On Ending". As `end` is a reserved
+        # keyword in Ruby, we are using `on_finishing` instead.
+        #
+        # Called when a {Span} is ending, after the end timestamp has been set
+        # but before span becomes immutable. This allows for updating the span
+        # by setting attributes or adding links and events.
+        #
+        # This method is called synchronously and should not block the current
+        # thread nor throw exceptions.
+        #
+        # This method is optional on the Span Processor interface. It will only
+        # get called if it exists within the processor.
+        #
+        # @param [Span] span the {Span} that just is ending (still mutable).
+        # @return [void]
+        def on_finishing(span); end
+
         # Called when a {Span} is ended, if the {Span#recording?}
         # returns true.
         #

sdk/test/opentelemetry/sdk/trace/span_processor_test.rb

@@ -15,6 +15,10 @@
     processor.on_start(span, context)
   end
 
+  it 'implements #on_finishing' do
+    processor.on_finishing(span)
+  end
+
   it 'implements #on_finish' do
     processor.on_finish(span)
   end