Remove restriction that SpanContext must be a final class, specify cr…

…eation. (open-telemetry#969)

* Remove restriction that SpanContext must be a final class

* Methods for SpanContext.

* Lint

* No bookmark?

* Root span needs sampling

* Non-root needs sampling too

* API-only

* Update specification/trace/api.md

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

* Update specification/trace/api.md

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

* Note

* less is more

* Update specification/trace/api.md

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

specification/trace/api.md

@@ -170,7 +170,6 @@ These functions MUST delegate to the `Tracing Context Utilities`.
 
 A `SpanContext` represents the portion of a `Span` which must be serialized and
 propagated along side of a distributed context. `SpanContext`s are immutable.
-`SpanContext` MUST be a final (sealed) class.
 
 The OpenTelemetry `SpanContext` representation conforms to the [W3C TraceContext
 specification](https://www.w3.org/TR/trace-context/). It contains two
@@ -192,6 +191,10 @@ of key-value pairs. TraceState allows multiple tracing
 systems to participate in the same trace. It is fully described in the [W3C Trace Context
 specification](https://www.w3.org/TR/trace-context/#tracestate-header).
 
+The API MUST implement methods to create a `SpanContext`. These methods SHOULD be the only way to
+create a `SpanContext`. This functionality MUST be fully implemented in the API, and SHOULD NOT be
+overridable.
+
 ### Retrieving the TraceId and SpanId
 
 The API MUST allow retrieving the `TraceId` and `SpanId` in the following forms: