Assume random trace ID and set th-field only for sampled spans

[oertl]

This PR reflects the latest agreements of the sampling SIG:

• assume least significant bits of trace ID are random by default
• remove th-field if not sampled

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[PeterF778]

Looks good!

[breedx-splk]

This is cool and really helps to clean a few things up along the way. Thanks! I do wonder if it's worth mentioning in the README that, under rare circumstances where someone has plugged in their own ID generator, they could possibly break consistent sampling.

There have been some fringe cases where users needed to bridge to an existing telemetry system while they were migrating to otel.