flow.log: Add built-in token sanitizing (for security/safety).

[ygoldfeld]

@ygoldfeld filed pre-open-source:

Something like -

• Msg_metadata take optional map of search/replace pairs, each one being string (to replace), String_view (to replace-with). And/or a filter function. Just make it elegant yet highly performant.
• Add some way to make it easy to invoke via FLOW_LOG_() API.
• Note usually the map is known at the log call site only, not at process startup or whatever. E.g., in (a certain server), it needed to replace a particular ConnectionID with "(redacted)" + same thing with potential hex version of that value.
• Have the replacement be done in a performant way at the last second probably by Ostream_log_msg_writer, while writing to final output ostream. Don't create a new copy of the string with everything replaced. (Recall, though, Logger is technically free to not use Ostream_log_msg_writer, so it just needs to be in Logger's formal contract to respect the filtering functionality.)

Priority that I know of: Nothing pressing at the moment at least. Came about due to some manual redacting like this that had to be done elsewhere.