Prohibit usage of enum value name strings in OTLP/JSON (#2758)

* Prohibit usage of enum value name strings in OTLP/JSON

Resolves #424

This change disallows using enum value names as strings
in OTLP/JSON and requires to use enum integer values only.

* Fix grammar

Co-authored-by: Bogdan Drutu <[email protected]>

specification/protocol/otlp.md

@@ -399,13 +399,25 @@ response headers when sending binary Protobuf encoded payload.
 
 JSON Protobuf encoded payloads use proto3 standard defined
 [JSON Mapping](https://developers.google.com/protocol-buffers/docs/proto3#json)
-for mapping between Protobuf and JSON, with one deviation from that mapping: the
-`trace_id` and `span_id` byte arrays are represented as
-[case-insensitive hex-encoded strings](https://tools.ietf.org/html/rfc4648#section-8),
-they are not base64-encoded like it is defined in the standard
-[JSON Mapping](https://developers.google.com/protocol-buffers/docs/proto3#json).
-The hex encoding is used for `trace_id` and `span_id` fields in all OTLP
-Protobuf messages, e.g. the `Span`, `Link`, `LogRecord`, etc. messages.
+for mapping between Protobuf and JSON, with the following deviations from that mapping:
+
+- The `trace_id` and `span_id` byte arrays are represented as
+  [case-insensitive hex-encoded strings](https://tools.ietf.org/html/rfc4648#section-8);
+  they are not base64-encoded like as it is defined in the standard
+  [Protobuf JSON Mapping](https://developers.google.com/protocol-buffers/docs/proto3#json).
+  The hex encoding is used for `trace_id` and `span_id` fields in all OTLP
+  Protobuf messages, e.g. the `Span`, `Link`, `LogRecord`, etc. messages.
+  For example, the `trace_id` field in a Span can be represented like this:
+  { "trace_id": "5B8EFFF798038103D269B633813FC60C", ... }
+
+- Values of enum fields MUST be encoded as integer values. Unlike the standard
+  [Protobuf JSON Mapping](https://developers.google.com/protocol-buffers/docs/proto3#json),
+  which allows values of enum fields to be encoded as either integer values or as enum
+  name strings, only integer enum values are allowed in OTLP JSON Protobuf Encoding;
+  the enum name strings MUST NOT be used.
+  For example, the `kind` field with a value of SPAN_KIND_SERVER in a Span can be
+  represented like this:
+  { "kind": 2, ... }
 
 Note that according to [Protobuf specs](
 https://developers.google.com/protocol-buffers/docs/proto3#json) 64-bit integer