See the Protocol Exporter doc:

If signals are sent that have no per-signal configuration from the previous point, OTEL_EXPORTER_OTLP_ENDPOINT is used as a base URL and the signals are sent to these paths relative to that:

• Traces: v1/traces

Fairly sure the issue is here:

if endpointURL.Path == "" {
	// Per spec, /v1/traces is the default:
	// (https://github.com/open-telemetry/opentelemetry-specification/blob/c14f5416605cb1bfce6e6e1984cbbeceb1bf35a2/specification/protocol/exporter.md#endpoint-urls-for-otlphttp)
	endpointURL.Path = "/v1/traces"
}

This contradicts Example 3 in the spec:

export OTEL_EXPORTER_OTLP_ENDPOINT=http://collector:4318/mycollector/

Traces are sent to http://collector:4318/mycollector/v1/traces

otel-cli will instead try sending the traces to http://collector:4318/mycollector, as the path is not empty. The collector I was sending traces to has a non-empty path in its URL, so this led to a bit of confusion.

Easy enough for us to work around it by giving the trace URL to otel-cli, but this is probably going to surprise other folk.

Unsure how this can be fixed without a breaking change (even if it's arguably a bug), hence bringing it up for discussion instead of trying to PR directly.