3.x: SessionStressTest sees 0 or negative open connections on ScyllaDB #1215

SessionStressTest.sessions_should_not_leak_connections fails against ScyllaDB at its first connection count (SessionStressTest.java:145-147):

java.lang.AssertionError: expected [4001] but found [0]    (first run)
java.lang.AssertionError: expected [4001] but found [-5]   (second run)

After 2000 concurrent cluster.connect() calls, manager.sessions.size() is 2000 as expected, but Metrics.getOpenConnections() reads 0, and on one run -5. The gauge (Metrics.java:82-93) sums HostConnectionPool.opened() over every pool, and opened() (HostConnectionPool.java:1020) sums one AtomicInteger per shard. A negative total means some per-shard counter was decremented more often than it was incremented. Where that happens is not yet established.

#250 reported the same assertion in 2023 (expected [2001] but found [0]) and was closed without a fix.

This is a long-group test, which CI never runs, so nothing flags it. HostConnectionPool is also edited by the open #917, #877 and #578.

How this was verified

Reproduced twice on scylla-3.x 5f3ac203d8, JDK 8, local CCM ScyllaDB 2026.2.2:

mvn -B verify -Plong -Dscylla.version=2026.2.2 -Dfmt.skip=true -Dclirr.skip=true \
  -Danimal.sniffer.skip=true -pl driver-core -Dtest=SessionStressTest

Found while measuring the long group's coverage for DRIVER-1001. Not checked against Cassandra.