Send submission latency metrics from the node

[mcrakhman]

For submission latency we would probably need to know two timestamps:

• when we got the tx on the node
• when we put it in the block

We exclude the time it took for the client to send the tx and for us to return a confirmation to the client. We should also measure this only for the node that accepted the tx via submit method called by the client directly, and not for intermediate nodes.

So this can require some bookkeeping (unless we find a better approach), thus this metric can e.g. only be collected when we introduce a specific parameter in the config.