core: improve ergonomics of the attribute API (Scala 3)

[iRevive]

Note

The new API will be available only for Scala 3.

These changes should address the existing 'verbosity' problem:

• Attributes constructor verbosity #1181
• Verbosity of addAttributes vs. Natchez #504

The new varargs API allows using Attribute, Option[Attribute], and Iterable[Attribute] in the same call.

---

Let's imagine we have the following setup:

val userIdKey = AttributeKey[String]("user.id")
val planKey   = AttributeKey[String]("plan")

// per-request dynamic attributes
val userId: Option[String] = Some("u-123")
val plan: Option[String]   = None
val extra = Attributes(Attribute("region", "us"), Attribute("env", "prod"))
val base  = Attribute("endpoint", "/checkout")

Currently, to create a span or record a metric, we need to do the following:

// combined attributes
val attrs: Attributes = {
  val builder = Attributes.newBuilder
  builder.addAll(userIdKey.maybe(userId))
  builder.addAll(planKey.maybe(plan))
  builder.addAll(extra)
  builder.add(base)
  builder.result()
}

// record ops
tracer.span("http.request", attrs)

With the new API, these attributes can be used directly:

tracer.span(
  "http.request", 
  userIdKey.maybe(userId),
  planKey.maybe(plan),
  extra,
  Attribute("endpoint", "/checkout")
)