feat: Add Bunny configuration for context propagation_style (open-telemetry#526)

open-telemetry/oteps#220 recommends:

>  For each message it accounts for, the "Deliver" or "Receive" span SHOULD link
to the message's creation context. In addition, if it is possible the creation
context MAY be set as a parent of the "Deliver" or "Receive" span.

But also acknolwedges:

> open-telemetry/opentelemetry-specification#454 To instrument pull-based
"Receive" operations as described in this document, it is necessary to add links
to spans after those spans were created. The reason for this is, that not all
messages are present at the start of a "Receive" operations, so links to related
contexts cannot be added at the start of the span.

Our "Receive" spans do not link to the message's creation context, and indeed it
doesn't seem possible to do so. Likewise for setting the parent.

In lieu of a specification layer solution, this proposes adding a configuration
flag so that users can opt-in to setting the "Send" context as parent for the
"Process" span. This comes at the cost of a continuous trace including the
"Send"-"Receive"-"Process" spans, with "Receive" orphaned from both "Send" and
"Process".

This doesn't attempt to reproduce the "none" `propagation_style` included in
`sidekiq` instrumentation and other similar gems. If this is an interesting
direction, we could consider implementing it.

Author: tamsin johnson

instrumentation/bunny/lib/opentelemetry/instrumentation/bunny/instrumentation.rb

@@ -21,6 +21,8 @@ class Instrumentation < OpenTelemetry::Instrumentation::Base
           defined?(::Bunny)
         end
 
+        option :propagation_style, default: :link, validate: %i[link child none]
+
         private
 
         def require_patches

instrumentation/bunny/lib/opentelemetry/instrumentation/bunny/patch_helpers.rb

@@ -34,7 +34,12 @@ def self.destination_name(exchange, routing_key)
 
         def self.extract_context(properties)
           # use the receive span as parent context
-          parent_context = OpenTelemetry.propagation.extract(properties[:tracer_receive_headers])
+          parent_context =
+            if Bunny::Instrumentation.instance.config[:propagation_style] == :child
+              OpenTelemetry.propagation.extract(properties[:headers])
+            else
+              OpenTelemetry.propagation.extract(properties[:tracer_receive_headers])
+            end
 
           # link to the producer context
           producer_context = OpenTelemetry.propagation.extract(properties[:headers])

instrumentation/bunny/test/opentelemetry/instrumentation/bunny/patches/queue_test.rb

@@ -12,6 +12,7 @@
 
 describe OpenTelemetry::Instrumentation::Bunny::Patches::Queue do
   let(:instrumentation) { OpenTelemetry::Instrumentation::Bunny::Instrumentation.instance }
+  let(:config) { {} }
   let(:exporter) { EXPORTER }
   let(:spans) { exporter.finished_spans }
 
@@ -30,7 +31,7 @@
     # Clear spans
     exporter.reset
 
-    instrumentation.install
+    instrumentation.install(config)
   end
 
   after do
@@ -72,5 +73,33 @@
       process_span = spans.find { |span| span.name == ".#{queue_name} process" }
       _(process_span).must_be_nil
     end
+
+    describe 'when propagation_style is child' do
+      let(:config) { { propagation_style: :child } }
+
+      it 'maintains a continuous trace' do
+        queue.publish('Hello, opentelemetry!')
+
+        queue.pop { |_delivery_info, _metadata, _payload| break }
+
+        send_span = spans.find { |span| span.name == ".#{queue_name} send" }
+        process_span = spans.find { |span| span.name == ".#{queue_name} process" }
+
+        _(process_span.parent_span_id).must_equal(send_span.span_id)
+        _(process_span.trace_id).must_equal(send_span.trace_id)
+      end
+
+      it 'propagates baggage' do
+        ctx = OpenTelemetry::Baggage.set_value('testing_baggage', 'it_worked')
+
+        OpenTelemetry::Context.with_current(ctx) do
+          queue.publish('Hello, opentelemetry!')
+        end
+
+        queue.pop do |_delivery_info, _metadata, _payload|
+          _(OpenTelemetry::Baggage.value('testing_baggage')).must_equal('it_worked')
+        end
+      end
+    end
   end
 end unless ENV['OMIT_SERVICES']