Ignore invalid SpanContext when adding Links (open-telemetry#1492)

* Ignore invalid spancontext in addLink

* SDK not API

* Changelog

Co-authored-by: Yuri Shkuro <yurishkuro@users.noreply.github.com>

CHANGELOG.md

@@ -7,6 +7,7 @@ release.
 
 ## Unreleased
 
+- Adds note that implementations can ignore links with invalid SpanContext([#1492](https://github.com/open-telemetry/opentelemetry-specification/pull/1492))
 - Adds `none` as a possible value for OTEL_TRACES_EXPORTER and OTEL_METRICS_EXPORTER to disable export
   ([#1439](https://github.com/open-telemetry/opentelemetry-specification/pull/1439))
 - Add [`ForceFlush`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#forceflush) to SDK's `TracerProvider` ([#1452](https://github.com/open-telemetry/opentelemetry-specification/pull/1452))

specification/trace/api.md

@@ -405,7 +405,8 @@ The Span creation API MUST provide:
   arguments. This MAY be called `AddLink`. This API takes the `SpanContext` of
   the `Span` to link to and optional `Attributes`, either as individual
   parameters or as an immutable object encapsulating them, whichever is most
-  appropriate for the language.
+  appropriate for the language. Implementations MAY ignore links with an
+  [invalid](#isvalid) `SpanContext`.
 
 Links SHOULD preserve the order in which they're set.