add user agent to OTLP exporter spec

The following PR adds a section to the OTLP exporter about setting a user agent. This was implemented in the otel collector and provides users with additional information to troubleshoot their telemetry pipelines with.

specification/protocol/exporter.md

@@ -160,4 +160,15 @@ Transient errors MUST be handled with a retry strategy. This retry strategy MUST
 
 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](otlp.md#failures). For the OTLP/gRPC, the full list of the gRPC retryable status codes can be found in the [gRPC response section](otlp.md#otlpgrpc-response).
 
+## User Agent
+
+OpenTelemetry protocol exporters SHOULD emit a User-Agent header to at a minimum identify the exporter, the language of its implementation, and the version of the exporter. For example, the Python OTLP exporter version 1.2.3 would report the following:
+
+```
+OpenTelemetry Exporter Python/1.2.3
+```
+
+The conventions used for specifying the language and version are available in the [Resource semantic conventions][resource-semconv].
+
+[resource-semconv]: ../resource/semantic_conventions/README.md#telemetry-sdk
 [otlphttp-req]: otlp.md#otlphttp-request