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.
The unified logging system (aka OSLog) expects a OSLogMessage and does
not allow just passing in a string variable. Instead the variable has to
be interpolated into a string literal, which sets the default privacy
setting to 'private' and hence reducts the whole message.
This commit sets the privacy setting to 'public'.
