otelcol.receiver.datadog: `trace_id_cache_size` is documented but not usable without an unexposed upstream feature gate

[nico1510]

Component(s)

• otelcol.receiver.datadog

What's wrong?

trace_id_cache_size is documented in Alloy as if it can be used directly on otelcol.receiver.datadog:

https://grafana.com/docs/alloy/latest/reference/components/otelcol/otelcol.receiver.datadog/

But in the upstream OpenTelemetry datadogreceiver, that cache only has an effect when the feature gate receiver.datadogreceiver.Enable128BitTraceID is enabled. Alloy does not appear to expose any way to enable that feature gate, so in stock Alloy this setting is effectively unusable.

Expected:
Either Alloy should expose a way to enable receiver.datadogreceiver.Enable128BitTraceID, or the docs should clearly say that trace_id_cache_size does not currently work in Alloy.

Actual:
The setting is documented and accepted in config, but there is no documented or supported way to make it take effect.

Steps to reproduce

1. Read the Alloy docs for otelcol.receiver.datadog, which document trace_id_cache_size.
2. Configure otelcol.receiver.datadog with trace_id_cache_size set.
3. Check how to enable the upstream receiver.datadogreceiver.Enable128BitTraceID feature gate in Alloy.
4. Observe that Alloy does not expose a way to enable that gate, even though upstream requires it for this setting to do anything.

System information

Any

Software version

Grafana Alloy v1.10.2

Configuration

otelcol.receiver.datadog "default" {
  trace_id_cache_size = 100

  output {
    metrics = []
    traces  = []
  }
}

Logs

N/A

[nico1510]

#3781 (comment) looks related