[Feature]: Document supported and unsupported OTEL variables

[BatmanAoD]

Related Problems?

As far as I can tell, the documentation for the opentelemetry_sdk crate doesn't say anywhere what environment variables it does or does not recognize. The only authoritative documentation appears to be this table, which is somewhat difficult to read given its width: https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix.md#environment-variables

My team believed that the Rust SDK supports all the standard OTEL environment variables (possibly because we didn't notice the "note" on this page, but I don't recall specifically how we came to this conclusion). We therefore relied on OTEL_TRACES_SAMPLER to configure our sampling, and consequently emitted 100x more traces than intended.

Describe the solution you'd like:

The opentelemetry_sdk crate itself should clearly document which environment variables are respected, and which are not.

Considered Alternatives

• The crate could link to the compliance matrix somewhere prominent in the documentation. It seems odd for me that the canonical source for this information would be part of the specification repo, though.
• The crate could support all environment variables. I think this should be the goal eventually.

Additional Context

No response

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[cijothomas]

Thanks! If you have bandwidth, please help by sending a PR to the sdk doc.rs. We do have some support, the best way to find is to search the repo, and document based on it.

we didn't notice the "note" on this page,

It's a relatively new addition to the doc, did not exist until last year! (open-telemetry/opentelemetry.io#4059)

[cijothomas]

opentelemetry-rust/opentelemetry-sdk/src/trace/config.rs

Line 60 in 07bb613

let sampler_arg = env::var("OTEL_TRACES_SAMPLER_ARG").ok();

Btw, we do seem to support OTEL_TRACES_SAMPLER..

[BatmanAoD]

@cijothomas Huh, the table says that the Rust SDK does not (...though I suppose it's not too surprising for them to fall out of sync) and our usage is bafflingly high.

Is there a way to actually assert that an SdkTracerProvider has a particular sampler configured, or even that a TracerProviderBuilder will use a specific sampler?

[BatmanAoD]

Okay, I've verified that the env vars are in fact respected. I don't have time for an MR to this repo at the moment but I'll open one in the spec repo to change those two cells in the table.