Trace latency: add optional OTLP export for local inspection

Parent: #281

# Goal

Add optional OTLP export for local trace inspection while working on the DataFusion latency path.

This is not a production latency optimization by itself. It gives engineers a way to inspect request spans, refresh spans, object-store behavior, and benchmark runs in a local tracing backend while tuning the system.

# Scope

- Add optional OTLP exporter configuration for local development.
- Keep the exporter off by default.
- Document the minimum environment variables needed to send traces to a local collector.
- Ensure the added spans from the benchmark and observability work are visible through OTLP.

# High-level design

This should be a small operational hook, not a new tracing architecture. The existing metrics and logs remain the source of truth for automated regression checks. OTLP exists so engineers can debug a bad run without reading raw logs line by line.

# Acceptance criteria

- OTLP export can be enabled by configuration.
- Default behavior is unchanged when OTLP settings are absent.
- A local request shows the high-level DataFusion query spans in an OTLP-compatible backend.
- Failure to reach the collector does not break request handling.