fix(redisotel): fix buggy append in reportPoolStats

[wzy9607]

The current append twice to conf.attrs approach in reportPoolStats may result in unexpected idleAttrs, due to append can mutate the underlying array of the original slice, as demonstrated at https://go.dev/play/p/jwRMofH91eQ?v=goprev and below.

Also, I replaced metric.WithAttributes in reportPoolStats with metric.WithAttributeSet, since WithAttributes is just WithAttributeSet with some extra works that are not needed here, see https://pkg.go.dev/go.opentelemetry.io/otel/[email protected]#WithAttributes.

demonstration of the bug

package main

import (
	"fmt"

	"go.opentelemetry.io/otel/attribute"
)

func main() {
	a := make([]attribute.KeyValue, 0, 4)
	a = append(a, attribute.String("a", "a"), attribute.String("b", "b"), attribute.String("c", "c"))
	idle := append(a, attribute.String("state", "idle"))
	used := append(a, attribute.String("state", "used"))
	fmt.Printf("a: %v\n", a)
	fmt.Printf("idle: %v\n", idle)
	fmt.Printf("used: %v\n", used)
}

outputs

a: [{a {4 0 a <nil>}} {b {4 0 b <nil>}} {c {4 0 c <nil>}}]
idle: [{a {4 0 a <nil>}} {b {4 0 b <nil>}} {c {4 0 c <nil>}} {state {4 0 used <nil>}}]
used: [{a {4 0 a <nil>}} {b {4 0 b <nil>}} {c {4 0 c <nil>}} {state {4 0 used <nil>}}]

while the intended result in reportPoolStats usecase is
idle: [{a {4 0 a <nil>}} {b {4 0 b <nil>}} {c {4 0 c <nil>}} {state {4 0 idle <nil>}}].