Metadata storage type & enable MDC storage use cases

[ktoso]

This is the "later" follow up ticket to the "take out metadata to start with and make opaque later #31".

One use case of being able to "take the entire metadata" is keeping around data that is set for the lifetime of a resource, so a library may want to "1) store it 2) run user code, allowing it to shadow or remove metadata etc 3) restore the metadata for the next invocation"

This pattern is sometimes useful, and would be nice if we could support it, esp since without a "give me the metadata" a framework can not implement this without knowing all the keys that it should keep+restore.

Example test:

    func testMDCStyleMetadata() {
        let testLogging = TestLogging()
        LoggingSystem.bootstrapInternal(testLogging.make)

        var logger = Logger(label: "\(#function)")
        logger[metadataKey: "always"] = "there"

        func userCode() {
            logger[metadataKey: "always"] = "other"
            logger.info("hello world!")

            testLogging.history.assertExist(level: .info, message: "hello world!", metadata: ["always": "other"])
        }

        let old = logger.metadata
        userCode()
        logger.metadata = old

        func userCode2() {
            logger.info("hello world!")
            testLogging.history.assertExist(level: .info, message: "hello world!", metadata: ["always": "there"])
        }
        userCode2()
    }