Add Timestamp to LogRecord for Tracing Bridge

[Jorgro]

When using tracing for logging it would be very nice to add the timestamp as per the specification.

It should be very low-effort to add here.

I noticed that this is done automatically for the Python logs so I can't see why the Rust SDK shouldn't do the same.

[cijothomas]

We set ObservedTimeStamp (it's done in the sdk, not in the bridge). Backends should be falling back to ObservedTimeStamp, if TimeStamp is not available.