runtime/pprof: set labelMap length

Change-Id: If09094e72161f2c5da9102706781524e32f87782
GitHub-Last-Rev: 89949bc
GitHub-Pull-Request: #54855
Reviewed-on: https://go-review.googlesource.com/c/go/+/428234
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: sashamelentyev

src/runtime/pprof/label.go

@@ -57,8 +57,8 @@ func (l *labelMap) String() string {
 // WithLabels returns a new context.Context with the given labels added.
 // A label overwrites a prior label with the same key.
 func WithLabels(ctx context.Context, labels LabelSet) context.Context {
-	childLabels := make(labelMap)
 	parentLabels := labelValue(ctx)
+	childLabels := make(labelMap, len(parentLabels))
 	// TODO(matloob): replace the map implementation with something
 	// more efficient so creating a child context WithLabels doesn't need
 	// to clone the map.