Add requirements for probability sampler (#611)

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

specification/trace/sdk.md

@@ -88,7 +88,7 @@ object must be immutable (multiple calls may return different immutable objects)
 
 Returns the sampler name or short description with the configuration. This may
 be displayed on debug pages or in the logs. Example:
-`"ProbabilitySampler{0.000100}"`.
+`"TraceIdRatioBased{0.000100}"`.
 
 Description MUST NOT change over time and caller can cache the returned value.
 
@@ -107,16 +107,29 @@ The default sampler is `ParentBased(root=AlwaysOn)`.
 * Returns `NOT_RECORD` always.
 * Description MUST be `AlwaysOffSampler`.
 
-#### Probability
+#### TraceIdRatioBased
 
-* The `ProbabilitySampler` MUST ignore the parent. To respect the parent
-`SampledFlag`, the `ProbabilitySampler` should be used as a delegate of the
-`ParentBased` sampler specified below.
-* Description MUST be `ProbabilitySampler{0.000100}`.
+* The `TraceIdRatioBased` MUST ignore the parent `SampledFlag`. To respect the
+parent `SampledFlag`, the `TraceIdRatioBased` should be used as a delegate of
+the `ParentBased` sampler specified below.
+* Description MUST be `TraceIdRatioBased{0.000100}`.
 
-TODO: Add details about how the `ProbabilitySampler` is implemented as a function
+TODO: Add details about how the `TraceIdRatioBased` is implemented as a function
 of the `TraceID`.
 
+##### Requirements for `TraceIdRatioBased` sampler algorithm
+
+* The sampling algorithm MUST be deterministic. A trace identified by a given
+`TraceId` is sampled or not independent of language, time, etc. To achieve this,
+implementations MUST use a deterministic hash of the `TraceId` when computing
+the sampling decision. By ensuring this, running the sampler on any child `Span`
+will produce the same decision.
+* A `TraceIdRatioBased` sampler with a given sampling rate MUST also sample all
+traces that any `TraceIdRatioBased` sampler with a lower sampling rate would
+sample. This is important when a backend system may want to run with a higher
+sampling rate than the frontend system, this way all frontend traces will
+still be sampled and extra traces will be sampled on the backend only.
+
 #### ParentBased
 
 * This is a composite sampler. `ParentBased` helps distinguished between the