Add timestamp to the KLoggingEvent

[vdshb]

When I comparing LoggingEvent from slf4j with KLoggingEvent I can see the last important missing part - TimeStamp (Maybe ThreadName is also important, but I don't really know how threads in Native/WASM world work. Maybe current coroutine name/dispatcher is worth to add as well).

It's a proposal to add a timestamp into KLoggingEvent.

I think the Instant from https://github.com/Kotlin/kotlinx-datetime is a good candidate for this field type. I don't know your thoughts about adding dependencies to the library, though. On the other hand, it's a decent and small KMP library by JB which might do the trick as adding oneliner:

public val timestamp: Instant? = Clock.System.now(),

It might also help with date/time formatting inside Formatter.

I assume payload might be another place to put timestamp into by custom logger... I haven't really get payload field's architecture purpose yet. But it's still worth to discuss separated field anyway, as using Map with String key for such popular field might cause performance issues (as well as adding Clock.System.now() in every logging event. I haven't really measured its performance, but I assume it's fast and not a problem).

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[oshai]

PR for adding the capability is welcome. few comments:

• it's better not to add dependencies. if the lib is small maybe we can reimplement the small parts required.
• timestamp should be added only to direct impl (not delegating loggers).
• adding something to every log event might get expensive :-) but if other libs did it, I guess we can do something similar.