If the initial update message would be too big for tokio-console's grpc
decoder, reduce the retention period and try again.

Currently the default retention period is 1 hour. That can easily grow
to more than the max grpc decode message size (4 MiB), at which point
tokio-console won't connect.

There's really no minimum safe duration for retention. It depends on how
busy the app is and on how much trace data runtime and tokio log. Here
we repeatedly divide the retention period in half until it fits in the
message.