pkg/scrape: Never block discovery with scrapes

[brancz]

The theory is that either scraping or pushing data might be keeping scrapers from updating their discovered targets. This holds target group locks for as short as possible and adds timeouts and the right contexts in various places.

[alwaysmeticulous]

✅ Meticulous spotted 0 visual differences across 274 screens tested: view results.

Meticulous evaluated ~4 hours of user flows against your PR.

_{Expected differences? Click here. Last updated for commit b86f59b pkg/scrape: Never block discovery with scrapes. This comment will update as new commits are pushed.}