OTel JS OTLP/protobuf exports to mockotlpserver result in a "Export succeeded but could not deserialize response" warning/error

[trentm]

% node -r @opentelemetry/auto-instrumentations-node/register simple-http-request.js
OTEL_LOGS_EXPORTER is empty. Using default otlp exporter.
OTEL_TRACES_EXPORTER is empty. Using default otlp exporter.
OpenTelemetry automatic instrumentation started successfully
incoming request: GET / {
  traceparent: '00-b9635b7dc1441966056992fa5e640134-ccc2827ccb01aff9-01',
  host: 'localhost:3000',
  connection: 'close'
}
client response: 200 {
  'content-type': 'text/plain',
  'content-length': '4',
  date: 'Thu, 12 Dec 2024 21:57:34 GMT',
  connection: 'close'
}
client response body: "pong"
OTLPExportDelegate Export succeeded but could not deserialize response - is the response specification compliant? RangeError: index out of range: 3 + 111 > 8
    at indexOutOfRange (/Users/trentm/el/elastic-otel-node/examples/node_modules/protobufjs/src/reader.js:13:12)
    at BufferReader.skip (/Users/trentm/el/elastic-otel-node/examples/node_modules/protobufjs/src/reader.js:343:19)
    at Reader.skipType (/Users/trentm/el/elastic-otel-node/examples/node_modules/protobufjs/src/reader.js:369:18)
    at Reader.skipType (/Users/trentm/el/elastic-otel-node/examples/node_modules/protobufjs/src/reader.js:373:22)
    at Function.decode (/Users/trentm/el/elastic-otel-node/examples/node_modules/@opentelemetry/otlp-transformer/build/src/generated/root.js:4268:48)
    at Object.deserializeResponse (/Users/trentm/el/elastic-otel-node/examples/node_modules/@opentelemetry/otlp-transformer/build/src/protobuf/serializers.js:59:34)
    at _promiseQueue.pushPromise._transport.send.then.resultCallback.code (/Users/trentm/el/elastic-otel-node/examples/node_modules/@opentelemetry/otlp-exporter-base/build/src/otlp-export-delegate.js:56:79)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) <Buffer 7b 22 6f 6b 22 3a 31 7d>

I think this started recently.

• Is this because protos are behind?
• Is the mockotlpserver server response an invalid response?
• Does this happen with gRPC? With HTTP/JSON?