Backport of VAULT-31409: trace postUnseal function into release/1.18.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #28895 to be assessed for backporting due to the inclusion of the label backport/1.18.x.

The below text is copied from the body of the original PR.

---

Description

This PR adds 2 new settings to the top-level Vault server configuration: enable_post_unseal_trace that when enabled will cause Vault to generate a Go trace during the execution of core.postUnseal and save it to disk under the directory configured in post_unseal_trace_dir, defaulting to /tmp/vault-traces/ instead if the field isn't set. This file can be inspected using the go tool trace command and will help us debug cases where the leadership transfer operation takes too long.

Most of the feature was split into a reusable StartDebugTrace function in a separate package. It could be used for similar debug purposes in the future but most importantly for now it avoids adding more code and dependencies to the already-bloated core package.

Manually tested the functionality to ensure that it was possible to generate the traces by updating the new fields in the config, then sending a SIGHUP signal to the running Vault process and triggering a leadership election via vault operator step-down. Disabling the generation of the traces via the same SIGHUP process also works. Post-unseal only runs on the active node so restarting a follower won't generate the post-unseal traces.

Jira: VAULT-31409

TODO only if you're a HashiCorp employee

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

---

Overview of commits

• a2c467c

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[bosouza]

CI green, no conflicts, should be good