feat: Propagate response attributes on Faraday::Error

The current `Faraday` middleware won't report `http.status_code` in
cases where an error is raised via another middleware (such as
`:raise_error` or `:json`).

This adds a special case for subclasses of `Faraday::Error` which have
response information available, so that `http.status_code` the span
status are still added to the span.

instrumentation/faraday/lib/opentelemetry/instrumentation/faraday/middlewares/tracer_middleware.rb

@@ -35,7 +35,11 @@ def call(env)
             ) do |span|
               OpenTelemetry.propagation.inject(env.request_headers)
 
-              app.call(env).on_complete { |resp| trace_response(span, resp) }
+              app.call(env).on_complete { |resp| trace_response(span, resp.status) }
+            rescue ::Faraday::Error => e
+              trace_response(span, e.response[:status]) if e.response
+
+              raise
             end
           end
 
@@ -62,9 +66,9 @@ def tracer
             Faraday::Instrumentation.instance.tracer
           end
 
-          def trace_response(span, response)
-            span.set_attribute('http.status_code', response.status)
-            span.status = OpenTelemetry::Trace::Status.error unless (100..399).cover?(response.status.to_i)
+          def trace_response(span, status)
+            span.set_attribute('http.status_code', status)
+            span.status = OpenTelemetry::Trace::Status.error unless (100..399).cover?(status.to_i)
           end
         end
       end

instrumentation/faraday/test/opentelemetry/instrumentation/faraday/middlewares/tracer_middleware_test.rb

@@ -178,5 +178,25 @@
         )
       end
     end
+
+    describe 'when faraday raises an error' do
+      let(:client) do
+        Faraday.new do |builder|
+          builder.response :raise_error
+          builder.adapter(:test) do |stub|
+            stub.get('/not_found') { |_| [404, {}, 'NOT FOUND'] }
+          end
+        end
+      end
+
+      it 'adds response attributes' do
+        assert_raises Faraday::Error do
+          client.get('/not_found')
+        end
+
+        _(span.attributes['http.status_code']).must_equal 404
+        _(span.status.code).must_equal OpenTelemetry::Trace::Status::ERROR
+      end
+    end
   end
 end