fix: metadata is not thread safe and has internal state

client/src/main/scala/spice4s/client/SpiceClient.scala

@@ -75,7 +75,7 @@ object SpiceClient {
  val METADATA_KEY = Metadata.Key.of(AUTHORIZATION, Metadata.ASCII_STRING_MARSHALLER)
 
  def fromChannel[F[_]: Async](channel: Channel, token: String): Resource[F, SpiceClient[F]] = {
- val metadata = {
+ def metadata = {
  val m = new Metadata()
  m.put(METADATA_KEY, s"Bearer ${token}")
  m