Define transient error in OTLP exporter specification (#3653)

Fixes #3652

This PR attempts to clearly define `transient error` in the OTLP
exporter specification.

It also fixes a bug in that the set of retryable status codes in the
OTLP exporter spec vs. the protocol spec are not aligned.

CHANGELOG.md

@@ -35,6 +35,11 @@ release.
 
 ### Resource
 
+### Protocol
+
+- Fix and clarify definition of "transient error" in the OTLP exporter specification.
+  ([#3653](https://github.com/open-telemetry/opentelemetry-specification/pull/3653))
+
 ### Compatibility
 
 - OpenTracing Shim: Allow invalid but sampled SpanContext to be returned.

specification/protocol/exporter.md

@@ -162,7 +162,22 @@ The `OTEL_EXPORTER_OTLP_HEADERS`, `OTEL_EXPORTER_OTLP_TRACES_HEADERS`, `OTEL_EXP
 
 Transient errors MUST be handled with a retry strategy. This retry strategy MUST implement an exponential back-off with jitter to avoid overwhelming the destination until the network is restored or the destination has recovered.
 
-For OTLP/HTTP, the errors `408 (Request Timeout)` and `5xx (Server Errors)` are defined as transient, detailed information about errors can be found in the [HTTP failures section](https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#failures). For the OTLP/gRPC, the full list of the gRPC retryable status codes can be found in the [gRPC response section](https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#otlpgrpc-response).
+### Transient errors
+
+Transient errors are defined by the
+[OTLP protocol specification][protocol-spec].
+
+For [OTLP/gRPC][otlp-grpc], transient errors are defined by a set of
+[retryable gRPC status codes][retryable-grpc-status-codes].
+
+For [OTLP/HTTP][otlp-http], transient errors are defined by:
+
+1. A set of [retryable HTTP status codes][retryable-http-status-codes] received
+   from the server.
+2. The scenarios described in:
+   [All other responses](https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#all-other-responses)
+   and
+   [OTLP/HTTP Connection](https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#otlphttp-connection).
 
 ## User Agent
 
@@ -177,3 +192,8 @@ The format of the header SHOULD follow [RFC 7231][rfc-7231]. The conventions use
 [resource-semconv]: ../resource/semantic_conventions/README.md#telemetry-sdk
 [otlphttp-req]: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#otlphttp-request
 [rfc-7231]: https://datatracker.ietf.org/doc/html/rfc7231#section-5.5.3
+[protocol-spec]: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md
+[otlp-grpc]: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#otlpgrpc
+[otlp-http]: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#otlphttp
+[retryable-grpc-status-codes]: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#failures
+[retryable-http-status-codes]: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#failures-1