logf: formalize temporary props

[jnichols-git]

My use of logf requires that I include HTTP request values (request ID, session ID) in log properties, but getting them out of context is verbose, so I'm passing the context as a property, then using the format to extract those values and delete the context prop. This works pretty well.

I don't want to encourage use of arbitrary objects as props to be output in format, but something like logf.Temp(name string, value any) that creates a prop visible to the formatter but not to the log output could allow for handy optional-value passing. Could work by prepending tmp_ to the name, then having the corresponding GetTemp do the same.