mod_measure_lua: add openmetrics-spirited way to collect lua memory use In the spirit of separate collectors found in other Prometheus packages, this is a separate module to collect metrics related to the lua stack itself.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```