Configure the OTel Collector probabilistic sampler processor for head-based sampling to reduce trace volume uniformly

Steps

1. Add the probabilistic_sampler processor to the Collector config under processors:
2. Set sampling_percentage to the desired percentage (e.g., 10 for 10%) and optionally set hash_seed for deterministic consistent sampling across Collector replicas
3. Place probabilistic_sampler after the memory_limiter and before exporters in the pipeline definition to avoid sampling before backpressure kicks in
4. Pair with the SDK-side parent-based sampler so spans whose parent was sampled always propagate as sampled regardless of the Collector rule
5. Validate with the debug exporter on a test pipeline to confirm the drop ratio before deploying to production