Enable opt-in google analytics for RustDocs. (#3762)

substrate/.gitlab-ci.yml

@@ -288,7 +288,7 @@ build-rust-doc-release:
   <<:                              *build-only
   script:
     - rm -f ./crate-docs/index.html # use it as an indicator if the job succeeds
-    - BUILD_DUMMY_WASM_BINARY=1 time cargo +nightly doc --release --all --verbose
+    - BUILD_DUMMY_WASM_BINARY=1 time RUSTDOCFLAGS="--html-in-header $(pwd)/rustdoc-header.html" cargo +nightly doc --release --all --verbose
     - cp -R ./target/doc ./crate-docs
     - echo "<meta http-equiv=refresh content=0;url=substrate_service/index.html>" > ./crate-docs/index.html
     - sccache -s

substrate/rustdoc-header.html

+<script defer type="text/javascript" src="https://substrate.dev/js/config.js"></script>
+<script defer type="text/javascript" src="https://substrate.dev/js/klaro.min.js"></script>
+<script async type="opt-in" data-type="application/javascript"
+		data-src="https://www.googletagmanager.com/gtag/js?id=UA-145158313-2" data-name="googleAnalytics"></script>
+<script type="opt-in" data-type="application/javascript" data-name="googleAnalytics">
+      window.dataLayer = window.dataLayer || [];
+      function gtag(){dataLayer.push(arguments);}
+      gtag('js', new Date());
+      gtag('config', 'UA-145158313-2');
+</script>