Add tail based tracing sampling

[komuw]

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

In #13 we added head based sampling.
But it would be nice to add sampling that would let us say something like;
Sample 5% of success but 100% of all the errors.

In

otero/tracing.go

Lines 64 to 79 in 903452e

	provider := trace.NewTracerProvider(
	trace.WithBatcher(exporter), // use batch in prod.
	trace.WithResource(resource),
	trace.WithSpanProcessor(loggingSpanProcessor{}),
	trace.WithSampler(
	// Sample 30%
	//
	// There's head-based sampling and tail-based sampling.
	// Tail based sampling would enable you to say something like;
	// `Sample 5% of success but 100% of all the errors.`
	//
	// What we have implemented here is head-based sampling.
	// See: https://github.com/komuw/otero/issues/11 (and the links therein)
	trace.ParentBased(trace.TraceIDRatioBased(0.3)),
	),
	)

• 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
• Discuss the possibility of deprecating the tail-based sampling processor open-telemetry/opentelemetry-collector-contrib#1797 (comment)
• 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/
• https://medium.com/@irisdyrmishi/tail-sampling-improve-your-tracing-platform-with-opentelemetry-86185308bcb8
• https://logz.io/learn/sampling-in-distributed-tracing-guide/