Add npm request forwarding to snowplow

changelogs/unreleased/40174-snowflake-npm-forward.yml

+---
+title: Measure npm request forwarding usage
+merge_request: 40174
+author:
+type: added

lib/api/helpers/packages/dependency_proxy_helpers.rb

@@ -10,6 +10,7 @@ module API
 
         def redirect_registry_request(forward_to_registry, package_type, options)
           if forward_to_registry && redirect_registry_request_available?
+            track_event("#{package_type}_request_forward")
             redirect(registry_url(package_type, options))
           else
             yield

spec/lib/api/helpers/packages/dependency_proxy_helpers_spec.rb

@@ -24,6 +24,7 @@ RSpec.describe API::Helpers::Packages::DependencyProxyHelpers do
 
     shared_examples 'executing redirect' do
       it 'redirects to package registry' do
+        expect(helper).to receive(:track_event).with('npm_request_forward').once
         expect(helper).to receive(:registry_url).once
         expect(helper).to receive(:redirect).once
         expect(helper).to receive(:fallback).never
@@ -63,6 +64,7 @@ RSpec.describe API::Helpers::Packages::DependencyProxyHelpers do
           let(:package_type) { pkg_type }
 
           it 'raises an error' do
+            allow(helper).to receive(:track_event)
             expect { subject }.to raise_error(ArgumentError, "Can't build registry_url for package_type #{package_type}")
           end
         end

spec/requests/api/npm_packages_spec.rb

@@ -88,12 +88,16 @@ RSpec.describe API::NpmPackages do
         it_behaves_like 'returning the npm package info'
 
         context 'with unknown package' do
+          subject { get api("/packages/npm/unknown") }
+
           it 'returns a redirect' do
-            get api("/packages/npm/unknown")
+            subject
 
             expect(response).to have_gitlab_http_status(:found)
             expect(response.headers['Location']).to eq('https://registry.npmjs.org/unknown')
           end
+
+          it_behaves_like 'a gitlab tracking event', described_class.name, 'npm_request_forward'
         end
       end