Clarify that unknown fields must be ignored when receiving OTLP/JSON

Resolves open-telemetry/opentelemetry-proto#425

The proposed behavior is necessary for interoperability of senders and receivers
when OTLP protocol evolves in an allowed way: by adding new fields to existing
messages.

CHANGELOG.md

@@ -29,7 +29,7 @@ release.
 - Add user agent to OTLP exporter specification
   ([#2684](https://github.com/open-telemetry/opentelemetry-specification/pull/2684))
 - Clarify that unknown fields must be ignored when receiving OTLP/JSON
-  ([#](https://github.com/open-telemetry/opentelemetry-specification/pull/))
+  ([#2816](https://github.com/open-telemetry/opentelemetry-specification/pull/2816))
 
 ### SDK Configuration
 

specification/protocol/otlp.md

@@ -424,7 +424,7 @@ for mapping between Protobuf and JSON, with the following deviations from that m
   This aligns with the behavior of the Binary Protobuf unmarshaler and ensures that adding
   new fields to OTLP messages does not break existing receivers.
   Protobuf unmarshalling implementations often support this via an option called
-  **Ignore unknown fields**.
+  "Ignore unknown fields".
 
 Note that according to [Protobuf specs](
 https://developers.google.com/protocol-buffers/docs/proto3#json) 64-bit integer