Add tracing sampling

[komuw]

In https://github.com/komuw/otero/blob/bb23f8a1f6228510f346e2824cb2eea888878cc6/tracing.go#L64-L71

There's head-based sampling and tail-based sampling.

• https://news.ycombinator.com/item?id=37295097 (control-find sampling)
• https://opentelemetry.io/docs/concepts/sampling/
• https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/processor/tailsamplingprocessor
• https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/1797#issuecomment-742477338
• https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/performance.md
• https://newrelic.com/blog/best-practices/open-telemetry-tail-sampling
• https://opentelemetry.io/docs/specs/otel/trace/tracestate-probability-sampling/

There is also filter processor that can be used in place of tail based sampling.

• https://tracetest.io/blog/opentelemetry-collectors-new-filter-processor
• https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/pkg/ottl/contexts/ottllog/README.md