Using otelcol.exporter.oltphttp to send logs to Loki OTLP points fails due to 204 response

[loafoe]

What's wrong?

Attempting to push logs from otelcol.exporter.olpthttp directly to a Loki 3.1.0 OTLP endpoint seems to fail i.e. logs are not acklowledged as being sent even though I can see the logs appear in Loki. This seems to be caused by Loki responding with a HTTP 204 which is not recognized as success by the oltphttp component.

I found a possible otel related upstream issue which was recently fixed:

open-telemetry/opentelemetry-cpp#2632
open-telemetry/opentelemetry-cpp#2712

If the otlphttp component is using this library then this would explain the above observation.

When I introduce a Caddy proxy in between with the following config, the problem goes away!

reverse_proxy /v1/logs {
    to loki-gateway.loki-system.svc:80
    @204 {
       status 204
    }
    handle_response @204 {
       copy_response 200
    }
    rewrite /otlp/v1/logs
}

Can alloy update to the latest opentelementry-cpp version to solve this?

Steps to reproduce

• Set up Alloy to push OTLP logs directly to a Loki 3.1.0 OTLP endpoint
• Observe the pipeline clogging up as delivery of logs is not acknowledged

System information

Linux grafana-alloy-1 6.1.92

Software version

Grafana Alloy v1.2.1

Configuration

loki.source.kubernetes "pods" {
  targets    = discovery.relabel.loki_relabel.output
  forward_to = [
    otelcol.receiver.loki.local.receiver,
  ]
}

otelcol.exporter.otlphttp "local" {
  client {
    # This endpoint rewrites /v1/logs to /otlp/v1/logs
    endpoint = "http://otlp-proxy.starlift-observability.svc"
    auth = otelcol.auth.headers.local.handler

    tls {
      insecure = false
    }
  }
}

Logs

ts=2024-07-12T08:18:33.463303168Z level=error msg="failed to consume log entries" component_path=/ component_id=otelcol.receiver.loki.local err="sending queue is full"

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