[SLG-0004]: Revised Metadata value attributes as CustomStringConvertible

[kukushechkin]

Important

This is a revision of SLG-0004 previously discussed here. Note this is a draft, including more than just the proposal implementation.

Add an extensible per-value attribute mechanism for metadata.

This PR also includes examples:

• Sensitivity attribute.
• OSLogHandler using Sensitivity attribute with os_log.

Motivation:

Metadata values in swift-log are opaque strings by the time the LogHandler receives them. The call site often
knows things about a value that the handler cannot infer — for example, whether the value should be redacted in
different environments.

Modifications:

• SLG-0004 proposal is updated based on the feedback.
• Added AttributedStringCarrier: CustomStringConvertible-based mechanism to Logging.
• Added Sensitivity as an example attribute to the new LoggingAttributes target.
• Added OSLogHandler as an example of a concrete log handler taking advantage of the Sensitivity attribute.
• Added benchmarks and benchmark thresholds for the new attributed metadata.

Result:

swift-log provides an optional extensible mechanism for attaching attributes to metadata values.