Extend LATENCY LATEST to add sum / cnt stats

[enjoy-binbin]

Currently LATENCY LATEST only has the following fields:

• Event name.
• Unix timestamp of the latest latency spike for the event.
• Latest event latency in millisecond.
• All-time maximum latency for this event.

This PR introduced these fields:

• sum: the all-time sum latency for this event (i think it is useful for some events)
• cnt: the event count that trigger the latency, with the sum we can calc the avg

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.02%. Comparing base (cda9eee) to head (eac30eb).
Report is 32 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1570      +/-   ##
============================================
+ Coverage     70.96%   71.02%   +0.05%     
============================================
  Files           120      121       +1     
  Lines         65094    65183      +89     
============================================
+ Hits          46191    46293     +102     
+ Misses        18903    18890      -13     

Files with missing lines	Coverage Δ
src/latency.c	81.21% <100.00%> (+0.29%)	⬆️

... and 28 files with indirect coverage changes

[zuiderkwast]

OK, I think we can add this.

We are adding these in the end of an array reply. I remember we did something like that in CLUSTER SLOTS and we said it is a backward-compatible change, but still it broke some clients. For example, hiredis-cluster was checking the exact array length in the handling of CLUSTER SLOTS.

LATENCY LATEST is not as risky to change as CLUSTER SLOTS, but we should still be aware that some monitoring tools maybe can break. We should mention it in the release notes and we should add some note in the documentation that new fields can be added. A note like that makes it more clear that adding more fields in the reply is a backward compatible change. Now we don't have a note like that on https://valkey.io/commands/latency-latest/.

[zuiderkwast]

@valkey-io/core-team Please vote to extend the multi-bulk reply with three more numbers.