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.

Sources/Logging OSLog/Logger Metadata Extension.swift

@@ -0,0 +1,41 @@
+import Logging
+
+extension Logging.Logger.Metadata {
+	public func asJSON() -> String {
+		return Self.asJSON(self)
+	}
+
+	private static func asJSON(_ metadata: Logging.Logger.Metadata) -> String {
+		var outputParts: [String: String] = [:]
+		for (key, value) in metadata {
+			let jsonKey = Self.escapeForJSON(key)
+			outputParts[jsonKey] = Self.asJSON(value)
+		}
+		return "{" + outputParts.map { "\"\($0)\": \($1)" }.joined(separator: ", ") + "}"
+	}
+
+	private static func asJSON(_ metadata: [Logging.Logger.MetadataValue]) -> String {
+		var outputParts: [String] = []
+		for item in metadata {
+			outputParts.append(Self.asJSON(item))
+		}
+		return "[" + outputParts.joined(separator: ", ") + "]"
+	}
+
+	private static func asJSON(_ metadata: Logging.Logger.MetadataValue) -> String {
+		switch metadata {
+		case .dictionary(let subvalues):
+			return Self.asJSON(subvalues)
+		case .array(let subvalues):
+			return Self.asJSON(subvalues)
+		case .string(let subvalue):
+			return "\"" + Self.escapeForJSON(subvalue) + "\""
+		case .stringConvertible(let subvalue):
+			return "\"" + Self.escapeForJSON("\(subvalue)") + "\""
+		}
+	}
+
+	private static func escapeForJSON(_ data: String) -> String {
+		return data.replacingOccurrences(of: "\"", with: "\\\"")
+	}
+}