Passing of a custom `MetadataKey` leads to an exception

[yevhenii-nadtochii]

There is a method io.spine.logging.ScopedLoggingContext.Builder.withMetadata(key: MetadataKey<T>, value: T) for attaching context metadata. It accepts a metadata key, which is an interface: interface MetadataKey<T: Any>. An attempt to create an object that implements this interface and pass it to the builder leads to an exception on JVM.

A failing code snippet:

import io.spine.logging.LoggingFactory
import io.spine.logging.MetadataKey
import io.spine.logging.WithLogging
import io.spine.logging.context.ScopedLoggingContext

typealias User = String

object UserKey : MetadataKey<User> {
    override val canRepeat: Boolean = false
    override val label: String = "user"
    override fun cast(value: Any): User =
        if (value is User) value else error("The passed `$value` can't be cast to `User`.")
}

object Example : WithLogging {
    fun doAct() {
        val user: User = "Bill"
        val logger = LoggingFactory.forEnclosingClass()
        ScopedLoggingContext.newContext()
            .withMetadata(UserKey, user)
            .execute {
                logger.atInfo().log { "..." }
            }
    }
}

fun main() = Example.doAct()

It fails with the following exception:

Exception in thread "main" java.lang.ClassCastException: class logging.test.UserKey cannot be cast to class io.spine.logging.JvmMetadataKey (logging.test.UserKey and io.spine.logging.JvmMetadataKey are in unnamed module of loader 'app')
        at io.spine.logging.context.DelegatingContextBuilder.withMetadata(LoggingContextFactory.kt:180)
        at logging.test.AppKt.main(App.kt:38)
        at logging.test.AppKt.main(App.kt)

However, it works fine if a key is created by LoggingFactory:

val userKey = LoggingFactory.singleMetadataKey("user", User::class)

We should either document somewhere that keys can be created only by the factory, or allow passing of custom ones.

---

Additionally, align naming of method arguments in overloads of singleMetadataKey(...) and repeatedMetadataKey(...). KClass for values is named different. We should go with valueClass name. It relates to LoggingFactory for JVM.