[trace/sampling] Revises ProbabilitySampler spec to avoid and/or erase ot.th key if trace ID randomness and an explicit rv value is not available.

Author: yuanyuanzhao3

specification/trace/sdk.md

@@ -506,32 +506,32 @@ to use ProbabilitySampler.
 **Status**: [Development](../document-status.md)
 
 The `ProbabilitySampler` implements simple, ratio-based probability sampling using randomness features specified in the [W3C Trace Context Level 2][W3CCONTEXTMAIN] Candidate Recommendation.
-OpenTelemetry follows W3C Trace Context Level 2, which specifies 56 bits of randomness,
-[specifying how to make consistent probability sampling decisions using 56 bits of randomness][CONSISTENTSAMPLING].
+W3C Trace Context Level 2 specifies 56 bits of randomness.
+OpenTelemetry follows that model and [specifies how to make consistent probability sampling decisions using the 56 bits of randomness][CONSISTENTSAMPLING].
 
-The `ProbabilitySampler` sampler MUST ignore the parent `SampledFlag`.
+The `ProbabilitySampler` MUST ignore the parent `SampledFlag`.
 For respecting the parent `SampledFlag`, see the `ParentBased` sampler specified below.
 
 Note that this is a non-composable form of probability
-sampler. `ProbabilitySampler` directly implements the SDKs Sampler
+sampler. `ProbabilitySampler` directly implements the SDK's Sampler
 API, whereas [`ComposableProbability`](#composableprobability) is the
 composable form for use with [`CompositeSampler`](#compositesampler).
 
 [W3CCONTEXTMAIN]: https://www.w3.org/TR/trace-context-2/
 
-##### `ProbabilitySampler` sampler configuration
+##### `ProbabilitySampler` configuration
 
-The `ProbabilitySampler` sampler is typically configured using a 32-bit or 64-bit floating point number to express the sampling ratio.
+The `ProbabilitySampler` is typically configured using a 32-bit or 64-bit floating point number to express the sampling ratio.
 The minimum valid sampling ratio is `2^-56`, and the maximum valid sampling ratio is 1.0.
-From an input sampling ratio, a rejection threshold value is calculated; see [consistent-probability sampler requirements][CONSISTENTSAMPLING] for details on converting sampling ratios into thresholds with variable precision.
+From the input sampling ratio, a rejection threshold value is calculated; see [consistent-probability sampler requirements][CONSISTENTSAMPLING] for details on converting sampling ratios into thresholds with variable precision.
 
 [CONSISTENTSAMPLING]: ./tracestate-probability-sampling.md
 
-##### `ProbabilitySampler` sampler algorithm
+##### `ProbabilitySampler` sampling algorithm
 
-Given a Sampler configured with a sampling threshold `T` and Context with randomness value `R` (typically, the 7 rightmost bytes of the trace ID), when `ShouldSample()` is called, it uses the expression `R >= T` to decide whether to return `RECORD_AND_SAMPLE` or `DROP`.
+Given a `ProbabilitySampler` configured with a sampling threshold `T` and Context with randomness value `R` (typically, the 7 rightmost bytes of the trace ID), when `ShouldSample()` is called, it uses the expression `R >= T` to decide whether to return `RECORD_AND_SAMPLE` or `DROP`.
 
-* If randomness value (R) is greater or equal to the rejection threshold (T), meaning when (R >= T), return `RECORD_AND_SAMPLE`, otherwise, return `DROP`.
+* If randomness value (R) is greater than or equal to the rejection threshold (T), meaning when (R >= T), return `RECORD_AND_SAMPLE`, otherwise, return `DROP`.
 * When (R >= T), the OpenTelemetry TraceState SHOULD be modified to include the key-value `th:T` for rejection threshold value (T), as specified for the [OpenTelemetry TraceState `th` sub-key][TRACESTATEHANDLING].
 
 [TRACESTATEHANDLING]: ./tracestate-handling.md#sampling-threshold-value-th
@@ -542,21 +542,22 @@ When a `ProbabilitySampler` makes a decision for a non-root Span
 based on TraceID randomness, there is a possibility that the TraceID
 was in fact generated by an older SDK, unaware of this specification.
 The Trace random flag lets us disambiguate these two cases.  This flag
-propagates information to let ProbabilitySampler Samplers confirm that
-TraceIDs are random, however this requires W3C Trace Context Level 2
-to be supported by every Trace SDK that has handled the context.
-
-When a ProbabilitySampler Sampler makes a decision for a non-root Span
+propagates information to let ProbabilitySampler confirm whether
+TraceIDs are random. For this disambiguation to work across the trace,
+each Trace SDK that has handled the context MUST propagate the trace
+flags from `traceparent` without dropping bits it does not interpret, so
+the Random flag can reach downstream samplers. An SDK does not need to
+implement all of W3C Trace Context Level 2 for this to work; forwarding
+the trace flags unchanged is sufficient.
+
+When a ProbabilitySampler makes a decision for a non-root Span
 using [TraceID randomness when the Trace random flag was not
-set](#presumption-of-traceid-randomness), the SDK SHOULD issue a
-warning statement in its log with a compatibility warning.  As an
-example of this compatibility warning:
-
-```
-WARNING: The ProbabilitySampler sampler is presuming TraceIDs are random
-and expects the Trace random flag to be set in confirmation.  Please
-upgrade your caller(s) to use W3C Trace Context Level 2.
-```
+set](#presumption-of-traceid-randomness) AND an explicit `rv` value is not available in `tracestate`,
+the SDK SHOULD NOT include the sampler's `th` value in the OpenTelemetry (`ot`) entry in `tracestate`.
+Furthermore, the sampler MUST erase any existing `th` sub-key from the
+OpenTelemetry (`ot`) `tracestate` entry.
+This is done so downstream processors and connectors can recognize this situation and MUST NOT infer statistical information
+from spans sampled using this method.
 
 #### ParentBased