Add observability tracking of KE<->Client connections

[davidv1992]

Closes #399

[codecov]

Codecov Report

❌ Patch coverage is 78.78788% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.97%. Comparing base (e248b94) to head (9892d24).

Files with missing lines	Patch %	Lines
nts-pool-ke/src/pool_ke.rs	77.55%	11 Missing ⚠️
nts-pool-ke/src/util.rs	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #413      +/-   ##
==========================================
+ Coverage   71.94%   71.97%   +0.02%     
==========================================
  Files          57       57              
  Lines       13213    13272      +59     
==========================================
+ Hits         9506     9552      +46     
- Misses       3707     3720      +13     

Flag	Coverage Δ
beta	72.17% <78.78%> (+0.03%)	⬆️
fuzz	60.91% <ø> (ø)
fuzz-client_request	35.35% <ø> (ø)
fuzz-key_exchange_response	41.30% <ø> (ø)
fuzz-record	35.93% <ø> (ø)
fuzz-server_information_response	24.97% <ø> (-0.47%)	⬇️
msrv	71.84% <78.78%> (-0.02%)	⬇️
stable	72.19% <78.78%> (+0.01%)	⬆️
test	71.91% <78.78%> (+0.02%)	⬆️
x86_64-unknown-linux-gnu	71.91% <78.78%> (+0.02%)	⬆️
x86_64-unknown-linux-musl	72.16% <78.78%> (-0.01%)	⬇️

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