fix: use lock for subsummary to get its values

[blindspotbounty]

There is a race condition listed in https://github.com/swift-server-community/SwiftPrometheus/issues/79

Checklist

• [x] The provided tests still run.
• [-] I've created new tests where needed.
• [-] I've updated the documentation if necessary.

Motivation and Context

That solves race-condition and corresponding crash from https://github.com/swift-server-community/SwiftPrometheus/issues/79

[ktoso]

Meh, wrong target branch -- we're developing on main, I'll pick it over though 🍒 ⛏️ :)

It seems we had the wrong target branch setting in github -- fixed.