otel: encourage allowing span links to be added after span creation (#…

…870)

The OTel spec has changed to allow span links to be added
after span creation. Our APM agent spec should allow following
suit.

specs/agents/span-links.md

@@ -35,5 +35,5 @@ If provided, the API and semantics SHOULD be compatible with the
 [OpenTelemetry specification on specifying span links](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#specifying-links). A compatible API will facilitate
 [OpenTelemetry bridge](trace-api-otel.md) support. OpenTelemetry requirements:
 
-- The public API MUST NOT allow adding span links *after* span creation.
+- The public API SHOULD allow adding span links *after* span creation.
 - Links SHOULD preserve the order in which they are set.