Fix: only SSH into explicitly configured hosts for ethtool collection

LinkStatsCollector.collect() was SSHing into every host reporting
node_network_* metrics to Prometheus, including unrelated app servers
like postgresql and matrix. Add instance_map membership check so ethtool
collection via Pulse only runs on hosts defined in config.json.

Prometheus metrics (traffic rates, errors) are still collected for all
instances — only the SSH/ethtool step is gated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

monitor.py

@@ -559,9 +559,11 @@ class LinkStatsCollector:
             host_ip = instance.split(':')[0]
             ifaces  = list(iface_metrics.keys())
 
-            # SSH ethtool collection via Pulse worker (one connection per host, all ifaces)
+            # SSH ethtool collection via Pulse worker — only for explicitly configured
+            # hosts (instance_map keys).  Hosts like postgresql/matrix may report
+            # node_exporter metrics to Prometheus but don't need link diagnostics.
             ethtool_data: Dict[str, dict] = {}
-            if self.pulse.url and ifaces:
+            if self.pulse.url and ifaces and instance in instance_map:
                 try:
                     ethtool_data = self._ssh_batch(host_ip, ifaces)
                 except Exception as e: