task: add tests for tracing instrumentation of tasks (#6112)

Tokio is instrumented with traces which can be used to analyze the
behavior of the runtime during execution or post-mortem. The
instrumentation is optional. This is where tokio-console collections
information.

There are currently no tests for the instrumentation.

In order to provide more stability to the instrumentation and prepare
for future changes, tests are added to verify the current behavior. The
tests are written using the `tracing-mock` crate. As this crate is still
unreleased, a separate test create has been added under `tokio/tests`
which is outside the workspace. This allows us to pull in both `tracing`
and `tracing-mock` from the tracing repository on GitHub without
affecting the rest of the tokio repository.

This change adds initial tests for the task instrumentation. Further
tests will be added in subsequent commits.

Once `tracing-mock` is published on crates.io (tokio-rs/tracing#539),
these tests can be moved in with the "normal" tokio integration tests.
The decision to add these tests now is due to the release of
`tracing-mock` taking a while, so it would be better to have tests while
we wait.

.github/workflows/ci.yml

@@ -44,6 +44,7 @@ jobs:
       - test-workspace-all-features
       - test-integration-tests-per-feature
       - test-parking_lot
+      - test-tracing-instrumentation
       - valgrind
       - test-unstable
       - miri
@@ -213,6 +214,34 @@ jobs:
       - name: Check tests with all features enabled
         run: cargo check --workspace --all-features --tests
 
+  test-tracing-instrumentation:
+    # These tests use the as-yet unpublished `tracing-mock` crate to test the
+    # tracing instrumentation present in Tokio. As such they are placed in
+    # their own test crate outside of the workspace.
+    needs: basics
+    name: test tokio instrumentation
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: ${{ env.rust_stable }}
+      - name: Install cargo-nextest
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-nextest
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: test tracing-instrumentation
+        run: |
+          set -euxo pipefail
+          cargo nextest run
+        working-directory: tokio/tests/tracing-instrumentation
+        env:
+          RUSTFLAGS: --cfg tokio_unstable -Dwarnings
+
   valgrind:
     name: valgrind
     needs: basics