Expand GoBot metrics and Grafana dashboard - #151

Summary

• preserve the existing seven global metrics while adding per-network, per-plugin, queue, process, and reliability metrics
• correct global connection health for multi-network deployments and accurately count commands handled by the first matching plugin
• enrich /stats with per-network operational details
• refresh the importable GoBot Operations dashboard for scrape health, IRC health, reliability, throughput, plugin activity, and queue pressure
• add filters matching the production job, environment, hostname, instance, and IRC network labels
• replace the old short Grafana notes with deployment, Prometheus validation, and import instructions
• add CI regression coverage for the dashboard JSON and remove the obsolete preview image

Validation

• go test -count=1 -race ./...
• go vet ./...
• go build ./...
• go test -cover ./...
• staticcheck ./...
• govulncheck ./... (no vulnerabilities found)
• focused gosec scan (0 actionable issues)
• jq -e . grafana/gobot-dashboard.json
• go mod verify
• go mod tidy -diff
• git diff --check