Links with invalid SpanContext are recorded. (#3928)

As Links with invalid SpanContext may contain attributes describing the
operation (specially useful and occurrent for messaging systems), we
still want to collect them. Fixes #2176 (in the issue extensive
discussion about took place and we came, back in the day, with an
initial agreement).

---------

Co-authored-by: Trask Stalnaker <trask.stalnaker@gmail.com>

CHANGELOG.md

@@ -11,6 +11,9 @@ release.
 
 ### Traces
 
+- Links with invalid SpanContext are recorded.
+  ([#3928](https://github.com/open-telemetry/opentelemetry-specification/pull/3928))
+
 ### Metrics
 
 - Change the exemplar behavior to be on by default.

specification/trace/api.md

@@ -805,8 +805,9 @@ The 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. Implementations MAY ignore links with an
-  [invalid](#isvalid) `SpanContext`.
+  appropriate for the language. Implementations SHOULD record links containing
+  `SpanContext` with empty `TraceId` or `SpanId` (all zeros) as long as either the attribute set
+  or `TraceState` is non-empty.
 
 The Span interface MAY provide: