marola.observability

Tracing backend for a local MLflow server (MAROLA_TRACES=mlflow, MIP-0010 §4.3/§5): spans go over OTLP/HTTP to /v1/traces with the x-mlflow-experiment-id header MLflow requires (its OTLP ingest endpoint, MLflow ≥ 3.6, takes an experiment id, not a name — so apply resolves /traces through the same experiments/get-by-name → create call the run ledger uses, MlflowApi.getOrCreateExperiment).