Log stack trace with warn message in ObservationDispatcher

Currently the ObservationDispatcher only logs the toString() of the exception of an event consumer in a warn message. One needs to enable debug level for ObservationDispatcher to get the stack trace of the exception. Most of the time it's quite difficult to track down the root cause of the exception without a stack trace. Enabling debug logging after the issue occurs is also not very convenient.

I suggest we print the stack trace of the exception with the warn message.