OpenTelemetry parent-based sampling

[anthonysalvato3]

Currently, the kong OpenTelemetry plugin supports AlwaysOn, AlwaysOff, and ratio-based sampling. In practice, parent-based sampling is useful for chaining together different sampling rules. For example, the otel parentbased_always_off sampler will always sample if there is a parent span in the trace, and never sample if there is not. Similarly, parentbased_traceidratio will always sample if there is a parent, and only sample at the specified ratio if there is no parent. This prevents situations where if service A calls service B and both do a standard ratio sample at 0.1, when a trace is started at service A, service B will only show up in the trace 10% of the time. Ideally, the 10% of the time service A starts the trace, service B will always show up.