refactor(syncthing): use slogutil expensive wrapper for CPU benchmark

[marbens-arch]

Purpose

Improves consistency and makes it a bit easier to change the log level to something else.

Testing

CPU benchmark looks like it works and has the same format as before.

2025-11-23 12:56:18 INF Measured hashing performance (perf="1017.53 MB/s" log.pkg=syncthing)

[calmh]

Yes possibly. This had me thinking a bit because it moves the computation to the logging framework, effectively, and I realized there is a bug because we format twice in different ways (internal + stdout), so

index 27844d546..e3f19a59e 100644
--- i/cmd/syncthing/main.go
+++ w/cmd/syncthing/main.go
@@ -51,6 +51,7 @@ import (
        "github.com/syncthing/syncthing/lib/locations"
        "github.com/syncthing/syncthing/lib/osutil"
        "github.com/syncthing/syncthing/lib/protocol"
+       "github.com/syncthing/syncthing/lib/rand"
        "github.com/syncthing/syncthing/lib/svcutil"
        "github.com/syncthing/syncthing/lib/syncthing"
        "github.com/syncthing/syncthing/lib/upgrade"
@@ -429,6 +430,12 @@ func (c *serveCmd) syncthingMain() {
                startPerfStats()
        }
 
+       slog.Info("Expensive stuff", "exp", slogutil.Expensive(func() any {
+               v := rand.String(8)
+               fmt.Println("computed:", v)
+               return v
+       }))
+
        // Print our version information up front, so any crash that happens
        // early etc. will have it available.
        slog.Info(build.LongVersion) //nolint:sloglint

% ../bin/syncthing --home h1                  
computed: H37xEnEX
computed: 74cKe2oH
2025-11-24 15:54:35 INF Expensive stuff (exp=74cKe2oH log.pkg=main)
2025-11-24 15:54:35 INF syncthing v2.0.12-dev.7.gf2e8d0f9.dirty-infrastructure "Hafnium Hornet" (go1.25.4 darwin-arm64) jb@jbo-m3wl72rv 2025-11-17 06:39:17 UTC [stnoupgrade] (log.pkg=main)

that should be fixed probably first...