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.

Author: tigrannajaryan

CHANGELOG.md

@@ -28,6 +28,8 @@ 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
+  ([#2816](https://github.com/open-telemetry/opentelemetry-specification/pull/2816))
 
 ### SDK Configuration
 

specification/protocol/otlp.md

@@ -419,6 +419,13 @@ for mapping between Protobuf and JSON, with the following deviations from that m
   represented like this:
   { "kind": 2, ... }
 
+- OTLP/JSON receivers MUST ignore message fields with unknown names and MUST unmarshal the
+  message as if the unknown field was not resent in the payload.
+  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".
+
 Note that according to [Protobuf specs](
 https://developers.google.com/protocol-buffers/docs/proto3#json) 64-bit integer
 numbers in JSON-encoded payloads are encoded as decimal strings, and either