otlp+json enums should be integers

[brettmc]

The otel collector currently accepts the string values of ENUMs for json-encoded payloads, but this is now prohibited and will likely be removed in a future collector release:

• Do NOT guarantee string enum values backwards compatibility in JSON opentelemetry-proto#424
• Prohibit usage of enum value name strings in OTLP/JSON opentelemetry-specification#2758

We currently use protobuf's serializeToJsonString() method to generate JSON payloads, which doesn't expose any options to represent enums as their integer values.

Possible solutions:

• submit an enhancement to protobuf PHP implementations (native + extension) to add/fix/expose the option to represent ENUMs as integers
• write a custom serializer for JSON

Additional context

Protobuf Ruby seems to support FormatEnumsAsIntegers
There is a partial implementation in the protobuf php extension for FormatEnumsAsIntegers, but it is not exposed via any public interfaces (and not available in the native library)

Tasks:

• update protobuf to allow enums as int
• update ProtobufSerializer to send new flag(s)
• wait for new version of protobuf to release

[brettmc]

PR against google/protobuf to allow formatting enums as strings for JSON output: protocolbuffers/protobuf#12707

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[brettmc]

Note that this has been worked around via #1192 until protobuf changes are accepted.