feat(sub): add socket-wide stats including dropped messages

[7suyash7]

Implement statistics tracking for the SubSocket, #37

Add a new SocketWideStats struct to hold metrics related to
the overall socket operation, distinct from per-publisher stats. Includes counters for:

• Messages dropped due to a full ingress buffer (driver -> socket)
• Messages successfully received by the socket frontend from the driver

[mempirate]

Thanks for the PR! I've got a small suggestion

[mempirate]

Agreed with having socket-agnostic stats for shared metrics, but I would like to see it implemented differently, something like the following:

struct SocketStats<S> {
   // ... socket-wide stats, and
   substats: S,
}

Where S = a stats struct specific to the socket implementation, for example SubStats or PubStats.

[7suyash7]

Hi, @mempirate yeah a unified structure makes sense. Just want to clarify though:
Are you thinking of a struct like maybe SocketStats<SpecificStats> (eg. SocketStats) where specific SpecificStats (like SubStats) hold the implementation specific details like the session map and the new counters like dropped_messages_total for Sub? And for this specific PR should I only refactor the SubSocket stats into this new pattern, leaving the refactor of Pub/Rep/Req stats for future work ? I'm happy to do a big refactor if thats preferred!

[mempirate]

Yeah that's right! And would be great if you'd want to do this across the different socket types so we don't get any inconsistencies

[7suyash7]

Hi! @mempirate, can you please take a look

[mempirate]

LGTM! Thanks for the contribution :)