Parent-based tracing sampling has incorrect logic -- should behave differently on flag=0 vs Parent Span doesn't exist

[andrejpk]

The current trace sampling behavior is not a good default. It conflates the parent trace flag and Trace ID-based sampling. With no config to configure something more specific, the default behavior should match the (OTel parent sampler)[https://github.com/open-telemetry/opentelemetry-go/blob/e6e186bfa485f679e35bb775cba63ca24029590d/sdk/trace/sampling.go#L161C1-L170C2].

In what area(s)?

/area runtime

What version of Dapr?

1.1.x

Expected Behavior

OTel ParentSampler behavior:

Parent sampling flag on -> sample
Parent sampling flag off -> do not sample
No/invalid parent: -> fall back to a root sampler (often traceIdRatioSampler)

Actual Behavior

Dapr Trace Sampler behavior:

Parent sampling flag on -> sample
Parent sampling flag off -> fall back to traceIdRatioSampler
No/invalid parent: -> fall back to traceIdRatioSampler

The current implementation combines the second two cases and calls traceIdRatioSampler. This leads to orphaned traces in observability systems, where a correct sampling decision is made for a trace in an upstream service but Dapr does not respect that decision and will start orphaned traces SamplingRate% of the time.

At a minimum, the default behavior should be changed to correctly implement the above logic (and use the ParentSampler in the Go Otel library instead of reimplementing this logic).

A more complete solution would be to expand the Sampling config to provide a richer configuration. For example, it is impossible right now to configure Dapr to only trace when a parent asks for tracing, because setting SamplingRate to 0 completely disables sampling, even though this a very common-sense configuration to use.

Steps to Reproduce the Problem

Release Note

RELEASE NOTE: