feat: Add add_link to span api/sdk

api/lib/opentelemetry/trace/span.rb

@@ -78,6 +78,28 @@ def add_attributes(attributes)
         self
       end
 
+      # Add a link to a {Span}.
+      #
+      # Adding links at span creation using the `links` option is preferred
+      # to calling add_link later, because head sampling decisions can only
+      # consider information present during span creation.
+      #
+      # Example:
+      #
+      #   span.add_link(OpenTelemetry::Trace::Link.new(span_to_link_from.context))
+      #
+      # Note that the OpenTelemetry project
+      # {https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/data-semantic-conventions.md
+      # documents} certain "standard attributes" that have prescribed semantic
+      # meanings.
+      #
+      # @param [OpenTelemetry::Trace::Link] the link object to add on the {Span}.
+      #
+      # @return [self] returns itself
+      def add_link(link)
+        self
+      end
+
       # Add an event to a {Span}.
       #
       # Example:

api/test/opentelemetry/trace/span_test.rb

@@ -36,6 +36,12 @@
     end
   end
 
+  describe '#add_link' do
+    it 'returns self' do
+      _(span.add_link(OpenTelemetry::Trace::Link.new(span_context))).must_equal(span)
+    end
+  end
+
   describe '#add_event' do
     it 'returns self' do
       _(span.add_event('event-name')).must_equal(span)

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

@@ -118,6 +118,37 @@ def add_attributes(attributes)
           self
         end
 
+        # Add a link to a {Span}.
+        #
+        # Adding links at span creation using the `links` option is preferred
+        # to calling add_link later, because head sampling decisions can only
+        # consider information present during span creation.
+        #
+        # Example:
+        #
+        #   span.add_link(OpenTelemetry::Trace::Link.new(span_to_link_from.context))
+        #
+        # Note that the OpenTelemetry project
+        # {https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/data-semantic-conventions.md
+        # documents} certain "standard attributes" that have prescribed semantic
+        # meanings.
+        #
+        # @param [OpenTelemetry::Trace::Link] the link object to add on the {Span}.
+        #
+        # @return [self] returns itself
+        def add_link(link)
+          @mutex.synchronize do
+            if @ended
+              OpenTelemetry.logger.warn('Calling add_link on an ended Span.')
+            else
+              @links ||= []
+              @links = trim_links(@links << link, @span_limits.link_count_limit, @span_limits.link_attribute_count_limit)
+              @total_recorded_links += 1
+            end
+          end
+          self
+        end
+
         # Add an Event to a {Span}.
         #
         # Example:
@@ -242,6 +273,7 @@ def finish(end_timestamp: nil)
             @end_timestamp = relative_timestamp(end_timestamp)
             @attributes = validated_attributes(@attributes).freeze
             @events.freeze
+            @links.freeze
             @ended = true
           end
           @span_processors.each { |processor| processor.on_finish(self) }
@@ -373,7 +405,7 @@ def trim_links(links, link_count_limit, link_attribute_count_limit) # rubocop:di
 
           if links.size <= link_count_limit &&
              links.all? { |link| link.span_context.valid? && link.attributes.size <= link_attribute_count_limit && Internal.valid_attributes?(name, 'link', link.attributes) }
-            return links.frozen? ? links : links.clone.freeze
+            return links
           end
 
           # Slow path: trim attributes for each Link.
@@ -386,7 +418,7 @@ def trim_links(links, link_count_limit, link_attribute_count_limit) # rubocop:di
             excess = attrs.size - link_attribute_count_limit
             excess.times { attrs.shift } if excess.positive?
             OpenTelemetry::Trace::Link.new(link.span_context, attrs)
-          end.freeze
+          end
         end
 
         def append_event(events, event) # rubocop:disable Metrics/CyclomaticComplexity, Metrics/PerceivedComplexity

sdk/test/opentelemetry/sdk/trace/span_test.rb

@@ -167,6 +167,47 @@
     end
   end
 
+  describe '#add_link' do
+    it 'does not add a link if span is ended' do
+      span.finish
+      span.add_link(OpenTelemetry::Trace::Link.new(context))
+      _(span.links).must_be_nil
+    end
+
+    it 'adds a link' do
+      span.add_link(OpenTelemetry::Trace::Link.new(context))
+      links = span.links
+      _(links.size).must_equal(1)
+      _(links.first.span_context).must_equal(context)
+    end
+
+    it 'adds a link with attributes' do
+      attrs = { 'foo' => 'bar' }
+      span.add_link(OpenTelemetry::Trace::Link.new(context, attrs))
+      links = span.links
+      _(links.size).must_equal(1)
+      _(links.first.attributes).must_equal(attrs)
+    end
+
+    it 'updates the links count' do
+      span.add_link(OpenTelemetry::Trace::Link.new(context))
+      span.add_link(OpenTelemetry::Trace::Link.new(context))
+      _(span.to_span_data.total_recorded_links).must_equal(2)
+    end
+
+    it 'trims excess links' do
+      span.add_link(OpenTelemetry::Trace::Link.new(context))
+      span.add_link(OpenTelemetry::Trace::Link.new(context))
+      _(span.links.size).must_equal(1)
+    end
+
+    it 'prunes invalid links' do
+      invalid_context = OpenTelemetry::Trace::SpanContext.new(trace_id: OpenTelemetry::Trace::INVALID_TRACE_ID)
+      span.add_link(OpenTelemetry::Trace::Link.new(invalid_context))
+      _(span.links.size).must_equal(0)
+    end
+  end
+
   describe '#add_event' do
     it 'add a named event' do
       span.add_event('added')