Exporting fails if a 204 No Content is returned

[gouthamve]

Description

The current http exporters expect a http.StatusOK (200) response code. But this is not required by the spec and also not something the collector requires.

Trying to use the exporter in a test fails with:

failed to upload metrics: failed to send metrics to http://localhost:9090/api/v1/otlp/v1/metrics: 204 No Content

[dmathieu]

There is actually a requirement to return a 200 from full success exports: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#full-success-1
The spec also mentions than any other unspecified HTTP response should conform to the HTTP specification: https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#all-other-responses

Which I read as: there shouldn't be a 204. But if there's one, we should handle it anyway.

and also not something the collector requires.

This link isn't about returning NoContent. It's about partial successes (which we do handle), and which still must return a 200.
See https://github.com/open-telemetry/opentelemetry-go/blob/main/exporters/otlp/otlpmetric/otlpmetrichttp/client.go#L195

With that said, I agree with you that we should handle NoContent. I've opened a PR for that.