Add warning or metric of long time between hearing of a block and adding it to the chain

[stringhandler]

Problem: A long time between the time a node first becomes aware of the existence of a block to the time it is added to the chain is a sign of a potential attack. Even if it is not an attack, there could be some inefficiencies in block validation or in IO to the block database.

I suggest we expose this time as a metric or warning log so that it can be displayed on a grafana dashboard and possibly trigger an alert.