tile38_expired_keys is never returned

[Maxim Kropotov]

Describe the bug
The metric for expired keys (tile38_expired_keys) is never returned from metrics endpoint

To Reproduce
Tile38 version: 1.32.1

1. run tile38 with Prometheus metrics, e.g ./tile38-server --metrics-addr localhost:1234
2. Set several keys with expiration, e.g. SET fleet truck POINT 33.5123 -112.2693 EX 1
3. Wait 1s for keys to expire, call metrics endpoint: http://localhost:1234/metrics

Expected behavior
Expect metrics to conain tile38_expired_keys, bit it doesn't

** Additional**
Created a PR mostly to illustrate what I understand the issue to be: #718
Feel free to close and /or redo if you prefer :-)