Output metadata as JSON

Metadata shall be machine readable (see SwiftLog/Structured Logging) and
the previous metadata serialization erased the nesting structure.
The JSON output ensures that the metadata can be quickly analyzed by either a
copy and paste into a JSON editor or a small CLI pipeline.

See https://swiftpackageindex.com/apple/swift-log/1.10.1/documentation/logging/002-structuredlogging
for more information on metadata.

Tests/Logging OSLog Tests/Logging Test.swift

@@ -13,4 +13,16 @@ import LoggingOSLog
 @Test func canLogMessage() {
 	let logger = Logging.Logger(label: "de.astzweig.loggingoslog.Test")
 	logger.info("Test message")
-}
+}
+
+@Test func canLogMessageWithMetadata() {
+	let logger = Logging.Logger(label: "de.astzweig.loggingoslog.Test")
+	logger.info("Test message", metadata: [
+		"request.id": "20140801",
+		"request.dirname": "\"Impossible\"",
+		"request.authorization": [
+			"bearer": "empty"
+		],
+		"request.values": ["1", "rootID", ["key": "value"]]
+	])
+}