in Sources/Logging/LogHandler.swift [140:149]
func log(level: Logging.Logger.Level, message: Logging.Logger.Message, metadata: Logging.Logger.Metadata?, file: String, function: String, line: UInt)
/// Add, remove, or change the logging metadata.
///
/// - note: `LogHandler`s must treat logging metadata as a value type. This means that the change in metadata must
/// only affect this very `LogHandler`.
///
/// - parameters:
/// - metadataKey: The key for the metadata item
subscript(metadataKey _: String) -> Logger.Metadata.Value? { get set }