[WIP] factor out trace functionality into separate package and add Go docs

util/semaphore/semaphore.go

@@ -3,9 +3,8 @@ package semaphore
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 	wsemaphore "golang.org/x/sync/semaphore"
-
-	"github.com/zrepl/zrepl/daemon/logging"
 )
 
 type S struct {
@@ -23,7 +22,7 @@ type AcquireGuard struct {
 
 // The returned AcquireGuard is not goroutine-safe.
 func (s *S) Acquire(ctx context.Context) (*AcquireGuard, error) {
-	defer logging.WithSpanFromStackUpdateCtx(&ctx)()
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	if err := s.ws.Acquire(ctx, 1); err != nil {
 		return nil, err
 	} else if err := ctx.Err(); err != nil {