fix: export generic PSU metrics

[rokernel]

Summary

• replace old PSU input/output numeric metrics with generic PSU voltage/current/power metrics
• skip missing, empty, N/A, and malformed PSU numeric values instead of exporting fake zeroes
• add Grafana dashboard, dashboard validation script, and docs updates for the new metric families

Testing

• ./scripts/validate-dashboard.sh dashboards/sonic-exporter.json
• go test ./internal/collector -run 'TestHwCollector|TestHwCollectorMetricFilter|TestHwCollectorPsuNumericMetricParsing' -count=1
• go test ./...
• go build ./...

Live validation

• Deployed an archive build to one SONiC switch that exposes generic PSU Redis fields
• Verified live metrics:
  • sonic_hw_psu_voltage_volts{slot="1"} 12.1
  • sonic_hw_psu_voltage_volts{slot="2"} 12.1
  • sonic_hw_psu_current_amperes{slot="1"} 19.968
  • sonic_hw_psu_current_amperes{slot="2"} 17.312
  • sonic_hw_psu_power_watts{slot="1"} 242
  • sonic_hw_psu_power_watts{slot="2"} 210
• sonic_hw_collector_success 1

Notes

• Validated on a second SONiC switch that did not expose usable generic PSU numeric fields; in that case the collector skipped the numeric metrics instead of exporting incorrect zeroes.
• After merge, the normal upstream release flow appears to be: annotate and push v0.1.2, then deploy the published GHCR image.