Observability: metrics, logging, and tracing

Summary

Implement observability infrastructure for apex: Prometheus metrics, structured logging, and OpenTelemetry tracing.

Metrics

Instrument all subsystems with OTel metrics exposed via Prometheus endpoint.

Sync engine

• apex_sync_head (gauge) — last synced height
• apex_sync_network_head (gauge) — upstream network head
• apex_sync_lag_seconds (gauge) — time behind network head
• apex_sync_backfill_duration (histogram) — per-batch backfill latency
• apex_sync_errors_total (counter) — sync errors by type
• apex_sync_backfill_progress_pct (gauge) — backfill completion percentage

API

• apex_rpc_request_duration (histogram) — per-method latency
• apex_rpc_request_total (counter) — per-method call count
• apex_rpc_errors_total (counter) — per-method errors

Store

• apex_store_query_duration (histogram) — SQLite query latency
• apex_store_insert_duration (histogram) — insert latency
• apex_store_size_bytes (gauge) — DB file size

Subscriptions

• apex_subscriptions_active (gauge) — active subscription count
• apex_subscription_deliveries (counter) — messages delivered
• apex_subscription_drops (counter) — messages dropped (slow reader)

Node

• apex_build_info (gauge) — with version labels
• apex_uptime_seconds (counter)

Logging

• Use slog (Go stdlib) — no heavy dependencies like ipfs/go-log
• Structured key-value pairs: height, namespace, method, duration, error
• Configurable log level at startup (and ideally at runtime via admin endpoint)

Tracing

• OpenTelemetry spans for sync fetch, store operations, and RPC handlers
• Configurable exporter (stdout for dev, OTLP for production)

API endpoint

Expose a unified SyncStatus() endpoint returning current synced height, network head, sync state (backfilling/streaming), lag, and active subscriptions in one call. celestia-node spreads this across 4 separate modules.

Configuration

[observability]
metrics_address = "0.0.0.0:9090"   # Prometheus endpoint
log_level = "info"                   # debug, info, warn, error
tracing_enabled = false
tracing_endpoint = ""                # OTLP endpoint

Empieza por mapear los puntos de entrada del motor de sincronización, la API, el almacén, las suscripciones, el nodo y la configuración, y después decide cómo encajan entre sí los requisitos de métricas, registro con slog y tracing. Verifica el endpoint de Prometheus, la respuesta SyncStatus, la configuración TOML y cada métrica y span indicados; la issue no menciona archivos ni tests, por lo que las ubicaciones de los tests deben identificarse durante la investigación.