fix: avoid using internal empty attribute.Set pointer

[rogercoll]

Description:

Data race condition when using the internal empty attribute.Set concurrently. Code changed to prevent using the internal attribute.Set instance.

Link to tracking Issue: #34882

Testing:
Same tests, race condition failures should be fixed.

Documentation:

[rogercoll]

Simplified example:

package main

import (
	"go.opentelemetry.io/otel/attribute"
	semconv "go.opentelemetry.io/otel/semconv/v1.21.0"
)

func customSet(anotherSet attribute.Set) (attribute.Set, error) {
	allAttributes := attribute.EmptySet()
	*allAttributes = attribute.NewSet(append(allAttributes.ToSlice(), anotherSet.ToSlice()...)...)
	return *allAttributes, nil
}

func main() {
	ch := make(chan bool, 2)
	go func() {
		_, _ = customSet(attribute.NewSet([]attribute.KeyValue{
			semconv.SourceAddress("1.2.3.4"),
		}...))
		ch <- true
	}()
	go func() {
		_, _ = customSet(attribute.NewSet([]attribute.KeyValue{
			semconv.SourceAddress("1.2.3.4"),
		}...))
		ch <- true
	}()
	<-ch
	<-ch
}

Check the race condition:

$ go run -race -v main.go

To fix it, replace the allAttributes := attribute.EmptySet() inti with allAttributes := &attribute.Set{}