Measure the chain publication latency

[masih]

Each chain exchange message is timestamped before being put to queue for publication. Add a metric at the point of actual publication to measure the time it took for the message to travel through the publication pipeline and be actually published.

This is easily done by getting the wall clock and and comparing it with the timestamp of the message just before actually publishing the message in chainexchange/pubsub.go

[BigLep]

2025-04-24: this isn't a problem currently so deferring.