Not able to use env var for otlp headers in config

Root Cause Analysis

I traced through the config and OTEL initialisation pipeline and found the exact location of the problem.

Where header values enter the pipeline:

codex-rs/core/src/config/types.rs defines OtelExporterKind with a plain headers: HashMap<String, String> field. Header values are stored verbatim from the TOML file — no interpolation step exists.

Where headers are consumed:

codex-rs/core/src/otel_init.rs converts the config.otel settings into an OtelExporter value via the to_otel_exporter closure. Both the OtlpHttp and OtlpGrpc branches simply clone header values straight through:

headers: headers
    .iter()
    .map(|(k, v)| (k.clone(), v.clone()))   // ← literal copy, no expansion
    .collect(),

What happens to ${OTLP_TOKEN}:

The string "${OTLP_TOKEN}" is passed unchanged all the way into the HTTP client header map and eventually sent as a literal header value, which is not a valid auth token.

Proposed fix:

Add a small expand_env_vars(value: &str) -> String helper in otel_init.rs that replaces ${VAR_NAME} with the corresponding process environment variable. If the variable is not set the placeholder is left unchanged (rather than silently becoming an empty string) so misconfiguration is visible.

fn expand_env_vars(value: &str) -> String {
    let mut result = String::with_capacity(value.len());
    let mut rest = value;
    while let Some(start) = rest.find("${") {
        result.push_str(&rest[..start]);
        rest = &rest[start + 2..];
        if let Some(end) = rest.find('}') {
            let var_name = &rest[..end];
            match std::env::var(var_name) {
                Ok(val) => result.push_str(&val),
                Err(_) => {
                    result.push_str("${");
                    result.push_str(var_name);
                    result.push('}');
                }
            }
            rest = &rest[end + 1..];
        } else {
            result.push_str("${");
        }
    }
    result.push_str(rest);
    result
}

Then change both map calls in to_otel_exporter from .map(|(k, v)| (k.clone(), v.clone())) to .map(|(k, v)| (k.clone(), expand_env_vars(v))).

This fix is entirely contained to otel_init.rs. No config schema changes are needed — users continue using headers = { ... } exactly as in the issue example. The change also covers OtlpGrpc headers and the trace_exporter/metrics_exporter variants since they all go through the same closure.

I've verified this compiles and added unit tests covering: substitution of a set variable, no-op for values with no placeholder, multiple placeholders in one value, an unset variable left unchanged, and an unclosed ${ left unchanged.

Happy to open a PR if that would be helpful.

Also observing the same; several rounds of trying to figure this out and it would appear that the docs are wrong and this does not work as expected.