Exporter should key beforeLease orchestration on configured before_lease hook

[mangelajo]

Summary

The current fix for #567/#569 still overloads LeaseContext.before_lease_hook with two different meanings:

1. beforeLease work has actually completed
2. cleanup should not deadlock

That is safe for the original no-hook deadlock, but it is still wrong for exporters that have a hook_executor without a configured before_lease hook, and it weakens ordering guarantees when a real before_lease hook is present.

Problem

serve() currently starts run_before_lease_hook() whenever self.hook_executor exists, even if the config only defines after_lease:

• serve() checks self.hook_executor
• run_before_lease_hook() then discovers config.before_lease is None, reports LEASE_READY, and sets before_lease_hook in finally

At the same time, handle_lease() only takes its no-hook fast path when self.hook_executor is None, and its finally block force-sets before_lease_hook before calling _cleanup_after_lease().

That creates two bugs:

1. after_lease-only exporters still run the background before-hook task even though there is no real before_lease hook configured.
2. before_lease_hook no longer strictly means beforeLease completion, so _cleanup_after_lease() can stop waiting for real before-hook completion once the finally path force-sets it.

Why this matters

after_lease-only configuration

With an after_lease hook but no before_lease hook:

• serve() still schedules run_before_lease_hook()
• handle_lease() does not do its no-hook LEASE_READY path because hook_executor exists
• if the lease ends early, cleanup can run before the background task finishes
• the background task can later report LEASE_READY after cleanup, potentially racing with the next lease

real before_lease configuration

When a real before_lease hook is configured and the lease ends while it is still running, handle_lease().finally can force-set before_lease_hook. _cleanup_after_lease() then stops waiting on the same event even though the real hook is still in progress.

Expected behavior

The exporter should key this behavior on whether a real before_lease hook is configured, not on whether hook_executor exists.

That means:

• only start run_before_lease_hook() when config.before_lease is present
• only use the no-hook LEASE_READY fast path when config.before_lease is absent
• only wait in _cleanup_after_lease() for real before-hook completion when config.before_lease is present
• only use the finally deadlock-escape set() for the no-before-hook path

Related

• Fix that introduced the current behavior: Fix exporter deadlock when lease ends before before_lease_hook is set #569
• Nearby cleanup change touching the same branch: fix: release lease on unused timeout when hooks are configured #598