OTel: Sampling support

Currently, the OTel integration (#189) uses the default `ParentBasedAlwaysOn` sampler — 100% of traces are captured. This is fine for initial rollout but we should add configurable sampling for production use.

## Options to consider

1. **Collector-side tail-based sampling** — keep errors and slow requests, drop routine 200s. No code changes needed, just collector config. This should be the first approach, and implies no code changes.
2. **SDK-side head sampling** — e.g. `TraceIdRatioBased(0.1)` for 10% sampling. Reduces overhead on the API process. Useful at very high request volumes.
3. **Per-route sampling** — different rates for different endpoints (e.g. 100% for webhooks, 1% for feature flags evaluation).

## Context

- The `BatchSpanProcessor` is lightweight, so SDK-side sampling is unlikely to be needed until high request volumes.
- Health/readiness endpoints are already excluded via `OTEL_TRACING_EXCLUDED_URL_PATHS`.
- This is also relevant for eventually replacing Sentry SDK tracing with OTel-native tracing.

Related: #182