SDK default samplers are underspecified

[arminru]

The spec lists a few default samplers but only mentions their names without specifying their behavior.

The ALWAYS_ON sampler, for example, does not specify if it only applies to root spans and must otherwise honor (i.e., replicate) the SampledFlag in the parent SpanContext's TraceFlags, or always return true, regardless of that flag.
Same goes with ALWAYS_OFF.

Java's AlwaysOnSampler, for example, always returns true:
https://github.com/open-telemetry/opentelemetry-java/blob/master/sdk/src/main/java/io/opentelemetry/sdk/trace/Samplers.java#L95

JS' ALWAYS_SAMPLER, for example, returns the parent's SampledFlag, if any, and only defaults to true if there is no parent:
https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-core/src/trace/sampler/ProbabilitySampler.ts#L29 (ALWAYS_SAMPLER = new ProbabilitySampler(1))