feat: Add untraced endpoints config to rack middleware (#594)

Author: robertlaurin

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

@@ -49,9 +49,15 @@ def clear_cached_config
 
           def initialize(app)
             @app = app
+            @untraced_endpoints = config[:untraced_endpoints].is_a?(Array) ? config[:untraced_endpoints] : []
           end
 
           def call(env) # rubocop:disable Metrics/AbcSize, Metrics/MethodLength
+            if untraced_request?(env)
+              OpenTelemetry::Common::Utilities.untraced do
+                return @app.call(env)
+              end
+            end
             original_env = env.dup
             extracted_context = OpenTelemetry.propagation.extract(
               env,
@@ -79,6 +85,10 @@ def call(env) # rubocop:disable Metrics/AbcSize, Metrics/MethodLength
 
           private
 
+          def untraced_request?(env)
+            @untraced_endpoints.include?(env['PATH_INFO'])
+          end
+
           # return Context with the frontend span as the current span
           def create_frontend_span(env, extracted_context)
             request_start_time = OpenTelemetry::Instrumentation::Rack::Util::QueueTime.get_request_start(env)

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

@@ -23,6 +23,7 @@
   let(:rack_builder) { Rack::Builder.new }
 
   let(:exporter) { EXPORTER }
+  let(:finished_spans) { exporter.finished_spans }
   let(:first_span) { exporter.finished_spans.first }
 
   let(:default_config) { {} }
@@ -71,6 +72,50 @@
       _(first_span.parent_span_id).must_equal OpenTelemetry::Trace::INVALID_SPAN_ID
     end
 
+    describe 'config[:untraced_endpoints]' do
+      describe 'when an array is passed in' do
+        let(:config) { { untraced_endpoints: ['/ping'] } }
+
+        it 'does not trace paths listed in the array' do
+          Rack::MockRequest.new(rack_builder).get('/ping', env)
+
+          ping_span = finished_spans.find { |s| s.name == '/ping' }
+          _(ping_span).must_be_nil
+
+          root_span = finished_spans.find { |s| s.name == '/' }
+          _(root_span).wont_be_nil
+        end
+      end
+
+      describe 'when a string is passed in' do
+        let(:config) { { untraced_endpoints: '/ping' } }
+
+        it 'traces everything' do
+          Rack::MockRequest.new(rack_builder).get('/ping', env)
+
+          ping_span = finished_spans.find { |s| s.name == '/ping' }
+          _(ping_span).wont_be_nil
+
+          root_span = finished_spans.find { |s| s.name == '/' }
+          _(root_span).wont_be_nil
+        end
+      end
+
+      describe 'when nil is passed in' do
+        let(:config) { { untraced_endpoints: nil } }
+
+        it 'traces everything' do
+          Rack::MockRequest.new(rack_builder).get('/ping', env)
+
+          ping_span = finished_spans.find { |s| s.name == '/ping' }
+          _(ping_span).wont_be_nil
+
+          root_span = finished_spans.find { |s| s.name == '/' }
+          _(root_span).wont_be_nil
+        end
+      end
+    end
+
     describe 'config[:allowed_request_headers]' do
       let(:env) { Hash('HTTP_FOO_BAR' => 'http foo bar value') }