add IsRemote field to SpanContext, set by propagators (open-telemetry#216)

Author: tsloughter

specification/api-propagators.md

@@ -24,6 +24,8 @@ Propagators API consists of two main formats:
 - `HTTPTextFormat` is used to inject and extract a value as text into carriers that travel
 in-band across process boundaries.
 
+Deserializing must set `IsRemote` to true on the returned `SpanContext`.
+
 ## Binary Format
 
 `BinaryFormat` is a formatter to serialize and deserialize a value into a binary format.

specification/api-sampling.md

@@ -30,7 +30,7 @@ Returns the sampling Decision for a `Span` to be created.
 - `SpanContext` of a parent `Span`. Typically extracted from the wire. Can be
   `null`.
 - Boolean that indicates that `SpanContext` was extracted from the wire, i.e.
-  parent `Span` is from the different process.
+  parent `Span` is from the different process, meaning `IsRemote` is set to true.
 - `TraceId` of the `Span` to be created.
 - `SpanId` of the `Span` to be created.
 - Name of the `Span` to be created.

specification/api-tracing.md

@@ -166,6 +166,9 @@ the same trace.
 `IsValid` is a boolean flag which returns true if the SpanContext has a non-zero
 TraceID and a non-zero SpanID.
 
+`IsRemote` is a boolean flag which returns true if the SpanContext was propagated 
+from a remote parent.
+
 Please review the W3C specification for details on the [Tracestate
 field](https://www.w3.org/TR/trace-context/#tracestate-field).
 
@@ -229,9 +232,8 @@ spans in the trace. Implementations MUST provide an option to create a `Span` as
 a root span, and MUST generate a new `TraceId` for each root span created.
 
 A `Span` is said to have a _remote parent_ if it is the child of a `Span`
-created in another process. Since the `SpanContext` is the only component of a
-`Span` that is propagated between processes, a `Span`'s parent SHOULD be a
-`SpanContext` if it is remote. Otherwise, it may be a `Span` or `SpanContext`.
+created in another process. Each propagators' deserialization must set 
+`IsRemote` to true so `Span` creation knows if the parent is remote.
 
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