RuntimeLogSink attaches a root
Handler that records
all log messages
Level.INFO or higher as a
RuntimePb.UPResponse.RuntimeLogLine
attached to the current
RuntimePb.UPResponse.
TODO: This class is designed to be used in a
single-threaded runtime. If multiple requests are executing in a
single process in parallel, their messages will currently overlap.
If we want to support this configuration in the future we should do
something slightly smarter here (however, we don't want to limit
logs to only the thread serving the request).