Observability

The three signals

• Logs — structlog JSON, shipped to Grafana Loki.
• Metrics — prometheus-fastapi-instrumentator, scraped by Grafana Cloud agent.
• Traces — OpenTelemetry OTLP → Grafana Tempo.

All three correlated by trace_id + correlation_id.

structlog setup

structlog.configure(
    processors=[
        structlog.contextvars.merge_contextvars,
        structlog.processors.add_log_level,
        structlog.processors.TimeStamper(fmt="iso"),
        structlog.processors.CallsiteParameterAdder([...]),
        inject_otel_span_ids,          # adds trace_id, span_id
        structlog.processors.JSONRenderer(),
    ],
    wrapper_class=structlog.make_filtering_bound_logger(logging.INFO),
)

Correlation

Middleware reads/creates , binds to + OTel baggage. Every log line in the request gets it automatically.