fix #717: tile38_expired_keys and several other metrics not returned

[undeadcat]

Please ensure you adhere to every item in this list

• [x] This PR was pre-approved by the project maintainer
• [x] I have self-reviewed the code
• [ ] I have added all necessary tests

Describe your changes

• statsExpired was never incremented
• tile38_expired_keys and some other metrics (seems like tile38_total_messages_sent) were never returned because int64 values were never handled int Prometheus endpoint (metrics.go).

This is my first PR here and in Go. Apologies if something is not typical or idiomatic :-) Feel free to close and rewrite if easier :-) Mostly wanted to illustrate the cause of the issue, as I understand it.

Issue number and link

Fixes https://github.com/tidwall/tile38/issues/717

[tidwall]

I'm good with your code as-is. Thanks!