scripts: add code coverage script

Add a script that collects overall code coverage profiles from all
tests (including the metamorphic test).

It generates three profiles, in LCOV format: `profile-tests.lcov`,
`profile-meta.lcov`, and `profile-combined.lcov`.

The intention is to make this script run nightly and add a front end
to visualize the coverage.

scripts/code-coverage.sh

@@ -0,0 +1,42 @@
+#!/bin/bash
+
+set -euxo pipefail
+
+mkdir -p artifacts
+
+tmpdir=$(mktemp -d)
+trap 'rm -rf "$tmpdir"' EXIT
+
+test_failed=0
+# The coverpkg argument ensures that coverage is not restricted to the tested
+# package; so this will get us overall coverage for all tests.
+go test ./... -coverprofile=artifacts/profile-tests.gocov -coverpkg=./... || test_failed=1
+
+# The metamorphic test executes itself for each run; we don't get coverage for
+# the inner run. To fix this, we use metarunner as the "inner" binary and we
+# instrument it with coverage (see https://go.dev/testing/coverage/#building).
+go build -o "${tmpdir}/metarunner" -cover ./internal/metamorphic/metarunner
+mkdir -p "${tmpdir}/metacover"
+
+GOCOVERDIR="${tmpdir}/metacover" go test ./internal/metamorphic \
+  -count 50 --inner-binary="${tmpdir}/metarunner" || test_failed=1
+
+go tool covdata textfmt -i "${tmpdir}/metacover" -o artifacts/profile-meta.gocov
+
+# TODO(radu): make the crossversion metamorphic test work.
+
+go run github.com/cockroachdb/code-cov-utils/[email protected] -out artifacts/profile-tests.lcov \
+  -trim-prefix github.com/cockroachdb/pebble/ \
+  artifacts/profile-tests.gocov
+
+go run github.com/cockroachdb/code-cov-utils/[email protected] -out artifacts/profile-meta.lcov \
+  -trim-prefix github.com/cockroachdb/pebble/ \
+  artifacts/profile-meta.gocov
+
+go run github.com/cockroachdb/code-cov-utils/[email protected] -out artifacts/profile-tests-and-meta.lcov \
+  -trim-prefix github.com/cockroachdb/pebble/ \
+  artifacts/profile-tests.gocov artifacts/profile-meta.gocov
+
+if [ $test_failed -eq 1 ]; then
+  echo "WARNING: some tests have failed; coverage might be incomplete."
+fi