Add a metric to display the latest block time

[freak12techno]

We have the following metric in place: cosmos_relayer_chain_latest_height, which obviously shows the latest chain height.
I use it internally to build alerts on (if it's stuck for like 5 minutes), but there might be cases where the chain is producing new blocks, but the node isn't in sync so cannot properly work.
I suggest adding another metric exposing a Unix timestamp of a latest block received, so it can also be used as an input for alerting.