feat: metrics agent for FreeSWITCH and Asterisk nodes
- pulse-lets-go-agent: collects metrics from PBX nodes via ESL (FreeSWITCH) or AMI (Asterisk)
- AMI client (raw TCP) for Asterisk Manager Interface
- Collector interface: collect, connect, subscribe hangup events, close
- FreeSWITCH collector: show channels, max_sessions, idle_cpu
- Asterisk collector: CoreShowChannels, CoreSettings
- System collector: /proc/loadavg, /proc/stat (CPU idle)
- Failure tracker: sliding window for call_failure_rate
- NATS publisher: pulse.metrics.<node_id> every 5 seconds
- Graceful shutdown, retry logic (5 failures → status=down)
- Template configs for UC nodes (Asterisk) and ses-sip (FreeSWITCH)
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