Clarify that lowerCamelCase field names MUST be used for OTLP/JSON

Resolves open-telemetry#2795

This is a breaking change for OTLP/JSON and is allowed because OTLP/JSON is not yet Stable.

CHANGELOG.md

@@ -34,6 +34,8 @@ release.
 
 ### OpenTelemetry Protocol
 
+- Clarify that lowerCamelCase field names MUST be used for OTLP/JSON
+ ([#2829](https:/open-telemetry/opentelemetry-specification/pull/2829))
 - Add user agent to OTLP exporter specification
  ([#2684](https:/open-telemetry/opentelemetry-specification/pull/2684))
 

specification/protocol/otlp.md

@@ -419,10 +419,18 @@ for mapping between Protobuf and JSON, with the following deviations from that m
  represented like this:
  { "kind": 2, ... }
 
+- The keys of JSON objects are field names converted to lowerCamelCase. Original
+ field names are not valid to use a keys of JSON objects.
+ For example this is a valid JSON representation of a Resource:
+ `{ "attributes": {...}, droppedAttributesCount: 123 }`, and this is NOT a valid
+ representation:
+ `{ "attributes": {...}, dropped_attributes_count: 123 }`.
+
 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
-numbers or strings are accepted when decoding.
+https://developers.google.com/protocol-buffers/docs/proto3#json):
+
+- 64-bit integer numbers in JSON-encoded payloads are encoded as decimal strings, and
+ either numbers or strings are accepted when decoding.
 
 The client and the server MUST set "Content-Type: application/json" request and
 response headers when sending JSON Protobuf encoded payload.