Add --debug-address flag for debug and profiling of the LSP

[emcfarlane]

Add --debug-address flag to buf lsp serve that starts a debug HTTP server (similar to gopls's --debug flag). When set, it serves a browsable debug UI with:

• Server info and build details
• Memory stats (runtime.MemStats)
• pprof endpoints (goroutines, heap, allocs, block, mutex)

Usage: start the LSP with --debug-address=localhost:0 and find the assigned port in the LSP logs. Open the address in a browser to inspect server state, memory usage, goroutines, and collect CPU/heap profiles to help debug issues.

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Apr 17, 2026, 3:42 PM

[emcfarlane]

Screenshots of the different views (and the familiar /debug/pprof handler).

[doriable]

This looks good to me, and I think that it will help us gather some better stats on the LSP. We can also iterate on this over time.

As for the flags, I was debating whether we wanted a default behaviour, e.g. --debug-address="", but this emulates the behaviour of gopls and other similar tooling, so I think having the user provide, at minimum, localhost:0 makes sense to me.

Approving ahead of merge conflict resolution.