As requested, it should be possible for Lua users to get the site's global id.

While this would thematically fit very well into Scribunto's Site library, only Wikibase currently knows a site's global id (which is caused by the current state of interwiki/site handling in MediaWiki).

I suggest adding mw.wikibase.getGlobalSiteId(). Alternatively, we could expose it simply as field mw.wikibase.globalSiteId or add it to Scribunto's Site library (although this would heavily mess with extension boundaries).

Note: T146616 is related to this as the $wgDBname/wfWikiId() matches the global site id in the WMF setup (but not generally).