message should be passed as a format-string and fields

[blueforesticarus]

Tracing is aimed toward structured logging. However the trace macros support format string syntax, which gets used often when it doesn't need to be. Ex) info!("server started on port: {}", port); should be info!(port, "server started");

Support for format syntax encourages people to use it, and is not necessary if you really need a formatted message. info!(message = format!("server starting on port {}", port)) is cumbersome enough that it won't be used as a default.

Meanwhile, the reason for using the format string in the first place is to control how your log looks visually. This is important and could be accomplished via a format = "server started on port: {port}" field, assuming your tracing subscriber supported it.
A reasonable default console layer would format the format string with the fields it specifies followed by a map of the other fields.

This would require custom formatting code, as std::fmt cannot be instantiated at runtime, and so would be slower than a normal use format!(...). The tracing macros could parse the format string at compile time, alleviating this.