fix: pending L7 events silently dropped with no observability

[mayankpande88]

Problem

When a TCP connect event hasn't been processed yet, L7 events queue in pendingL7Events (capped at 500 in registry.go). If the TCP event never arrives (lost from perf buffer), those L7 events are silently dropped after retry.

No metric tracks:

• How many events are pending
• How many events were dropped due to missing TCP connection
• How many events were dropped due to the 500 cap

Fix

1. Add a node_agent_pending_l7_events gauge
2. Add a node_agent_l7_events_dropped_total counter with reason label (no_connection, cap_exceeded, retry_exhausted)

Impact

Medium — silent data loss with no operator visibility.