Unique session/client tracking using HyperLogLog (#884)

## Summary

Adds probabilistic cardinality estimation for tracking unique HTTP clients and sessions using a HyperLogLog implementation.

- Add a `HyperLogLog<Precision>` template in `zentelemetry` with thread-safe lock-free register updates, merge support, and XXH3 hashing
- Feed client IP addresses (via raw bytes) and session IDs (via `Oid` bytes) into their respective HyperLogLog estimators from both the ASIO and http.sys server backends
- Emit `distinct_clients` and `distinct_sessions` cardinality estimates in HTTP `CollectStats()`
- Add tests covering empty, single, duplicates, accuracy, merge, and clear scenarios

## Why HyperLogLog

Tracking exact unique counts would require storing every observed IP or session ID. HyperLogLog provides a memory-bounded probabilistic estimate (~1–2% error) using only a few KB of memory regardless of traffic volume.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/zenhttp/httpserver.cpp b/src/zenhttp/httpserver.cpp
index e5cfbcbae..a46c5b851 100644
--- a/src/zenhttp/httpserver.cpp
+++ b/src/zenhttp/httpserver.cpp
@@ -988,6 +988,9 @@ HttpServer::CollectStats()
 	}
 	Cbo.EndObject();
 
+	Cbo << "distinct_clients" << m_ClientAddresses.Count();
+	Cbo << "distinct_sessions" << m_ClientSessions.Count();
+
 	Cbo.BeginObject("websockets");
 	{
 		Cbo << "active_connections" << GetActiveWebSocketConnectionCount();