Expose a metric that would measure the time taken from receiving a message and sending it back

[yuzhouchen-instaclustr]

Currently the metric shotover_chain_latency does not take into account the time it takes to encode each message, it would be better if we can include that time as well.

[conorbros]

Just to clarify, you mean include the time it takes to decode the message on arrival, process through the chain and encode at the end?

[rukai]

I investigated this yesterday/today and have a design that I'll go ahead with:

• at the decode codecs: include timestamp in Message (or maybe Messages)
• at the sink encode codec: histogram!(sink_to_source_latency, sink=name_of_sink)
• at the source encode codec: histogram!(source_to_sink_latency, source=name_of_source)

[rukai]

completed in #1395