test failed in CI: oximeter-collector agent::tests::test_self_stat_error_counter

[smklein]

This test failed on a CI run on #10500:

https://github.com/oxidecomputer/omicron/pull/10500/checks?check_run_id=78342847644

Log showing the specific test failure:

https://buildomat.eng.oxide.computer/wg/0/details/01KSQR0FVX669WDC81SJZ98AA5/mccBgeWXWTJhk9RLWazrSDONQwOzMdAI6ljC2LYBMSZhSA53/01KSQR0XRSZVQ8QS7MABQ96CK0#S9079

Excerpt from the log showing the failure:

2026-05-28T17:23:27.462Z	    thread 'agent::tests::test_self_stat_error_counter' (2) panicked at oximeter/collector/src/agent.rs:1020:14:
2026-05-28T17:23:27.462Z	    called `Option::unwrap()` on a `None` value