Add correlation id in X-Request-ID for external pipeline validation

This will allow sharing correlation id between the external pipeline
validation service and GitLab.

This is the second try because the previous one was reverted
because of a broken-master.

changelogs/unreleased/326106-correlation-id-to-validation-service.yml

+---
+title: Add correlation id in X-Request-ID for external pipeline validation
+merge_request: 58741
+author:
+type: other

lib/gitlab/ci/pipeline/chain/validate/external.rb

@@ -69,8 +69,10 @@ module Gitlab
             end
 
             def validate_service_request
-              headers = {}
-              headers['X-Gitlab-Token'] = validation_service_token if validation_service_token
+              headers = {
+                'X-Request-ID' => Labkit::Correlation::CorrelationId.current_id,
+                'X-Gitlab-Token' => validation_service_token
+              }.compact
 
               Gitlab::HTTP.post(
                 validation_service_url, timeout: validation_service_timeout,

spec/lib/gitlab/ci/pipeline/chain/validate/external_spec.rb

@@ -57,13 +57,14 @@ RSpec.describe Gitlab::Ci::Pipeline::Chain::Validate::External do
     before do
       stub_env('EXTERNAL_VALIDATION_SERVICE_URL', validation_service_url)
       allow(Gitlab).to receive(:com?).and_return(dot_com)
+      allow(Labkit::Correlation::CorrelationId).to receive(:current_id).and_return('correlation-id')
     end
 
     it 'respects the defined payload schema' do
       expect(::Gitlab::HTTP).to receive(:post) do |_url, params|
         expect(params[:body]).to match_schema('/external_validation')
         expect(params[:timeout]).to eq(described_class::DEFAULT_VALIDATION_REQUEST_TIMEOUT)
-        expect(params[:headers]).to eq({})
+        expect(params[:headers]).to eq({ 'X-Request-ID' => 'correlation-id' })
       end
 
       perform!
@@ -127,7 +128,7 @@ RSpec.describe Gitlab::Ci::Pipeline::Chain::Validate::External do
 
       it 'passes token in X-Gitlab-Token header' do
         expect(::Gitlab::HTTP).to receive(:post) do |_url, params|
-          expect(params[:headers]).to eq({ 'X-Gitlab-Token' => '123' })
+          expect(params[:headers]).to include({ 'X-Gitlab-Token' => '123' })
         end
 
         perform!