fix(cache): ensure LoadingOnceCache can only load once

[maxbrunet]

Why?

Same as parca-dev/parca#3768

What?

🤖 Generated by Copilot at 68d934c

Introduced a new cache type to prevent duplicate loading of data and improved cache interface naming in pkg/cache/loading.go.

How?

🤖 Generated by Copilot at 68d934c

• Rename Get method of LoadingLRUCacheWithTTL to getOrLoad and add a wrapper Get method to preserve original interface (link, link)
• Add LoadingOnceCache type that embeds LoadingLRUCacheWithTTL and adds singleflight logic to coordinate concurrent requests for the same key (link)
• Modify NewLoadingOnceCache function to return a pointer to LoadingOnceCache and initialize embedded cache and singleflight group (link)
• Add Get method to LoadingOnceCache that overrides embedded cache's Get method and uses singleflight group to memoize and return the value of the first call for the same key (link)

Test Plan

Unit tests

[kakkoyun]

Awesome 🏆 🥇 💯