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

…(#2816)

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

Resolves open-telemetry#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.

* Remove unnecessary sentence

* Fix typo

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

specification/protocol/otlp.md

@@ -419,6 +419,11 @@ 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 present 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.
+
 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