Run coverage in CI

Generate a coverage report in CI with Bazel. This requires using Linux
and version of GCC that works with the Bazel GCov parser[1]. In this
case, CI uses Ubuntu 18.04 with GCC 7.

[1]: bazelbuild/bazel#11538

.github/workflows/main.yml

@@ -99,3 +99,54 @@ jobs:
         find include src tests \(  -name "*.cc" -o -name "*.h" \) -exec clang-format -i {} \;
         git update-index --really-refresh
         git --no-pager diff
+
+  coverage:
+    runs-on: ubuntu-18.04
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Setup Bazel
+      env:
+          BAZEL_VERSION: "3.4.1"
+      run: |
+        sysctl kernel.unprivileged_userns_clone=1
+        sudo apt-get update -q
+        sudo apt-get install libxml2-utils -y
+        OS=linux
+
+        URL="https://github.com/bazelbuild/bazel/releases/download/${BAZEL_VERSION}/bazel-${BAZEL_VERSION}-installer-${OS}-x86_64.sh"
+
+        wget -O install.sh "${URL}"
+        chmod +x install.sh
+        ./install.sh --user
+        rm -f install.sh
+
+        cat .bazelrc.ci >> .bazelrc
+
+        echo "::set-env name=CXX::$CC"
+
+    - name: Checkout Submodules
+      run: |
+        git submodule update --init --recursive --recommend-shallow
+
+    - name: Build coverage report
+      run: |
+        bazel coverage //...
+
+    - name: Install lcov
+      run: |
+        sudo apt-get install -y lcov
+
+    - name: Print coverage summary
+      run: |
+        lcov --list bazel-out/_coverage/_coverage_report.dat
+
+    - uses: actions/upload-artifact@v2
+      with:
+        name: coverage-report
+        path: bazel-out/_coverage/_coverage_report.dat
+
+#    - uses: codecov/codecov-action@v1
+#      with:
+#        file: ./build/coverage.info
+#        fail_ci_if_error: true