ipx-directory traces, and ipx's trace follows its calls there (#213)

The exporter setup and the traceparent parser move from ipx into ipx_directory::otel, which ipx
already depends on, so both binaries share them. The directory makes a span a request, a child of
the caller's traceparent, and a trace a refresher round with a span a feed; ipx sends the current
span's traceparent on every call to the directory and wraps each share in a span. The dashboard's
Traces row and the prod-check skill cover all three services.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/directory/src/otel.rs

@@ -0,0 +1,83 @@
+//! Traces, for ipx and the directory both (#213): the exporter, and W3C `traceparent` in from a
+//! caller and out to a callee, so one trace follows a request from the browser through ipx-web
+//! and ipx into the directory.
+
+/// Traces over OTLP, to Tempo for one, when OTEL_EXPORTER_OTLP_ENDPOINT names a collector
+/// (`http://host:4318`: the exporter speaks OTLP over HTTP and adds `/v1/traces`). The exporter
+/// reads that and the other `OTEL_` variables itself.
+pub fn provider(service: &'static str) -> anyhow::Result<Option<opentelemetry_sdk::trace::SdkTracerProvider>> {
+    if std::env::var("OTEL_EXPORTER_OTLP_ENDPOINT").unwrap_or_default().is_empty() {
+        return Ok(None);
+    }
+    let exporter = opentelemetry_otlp::SpanExporter::builder().with_http().build()?;
+    let resource = opentelemetry_sdk::Resource::builder().with_service_name(service).build();
+    Ok(Some(
+        opentelemetry_sdk::trace::SdkTracerProvider::builder()
+            .with_batch_exporter(exporter)
+            .with_resource(resource)
+            .build(),
+    ))
+}
+
+/// The trace a request from the page belongs to, from its `traceparent` (#190), so the requests
+/// one action sets off are one trace. None for a missing or malformed one. In the query where no
+/// header can be set: a position saved by sendBeacon, and the playlist Safari's player fetches
+/// itself (#192).
+pub fn remote_parent(headers: &axum::http::HeaderMap, query: Option<&str>) -> Option<opentelemetry::Context> {
+    use opentelemetry::trace::{SpanContext, SpanId, TraceContextExt, TraceFlags, TraceId, TraceState};
+    let v = match headers.get("traceparent") {
+        Some(h) => h.to_str().ok()?,
+        None => query?.split('&').find_map(|kv| kv.strip_prefix("traceparent="))?,
+    };
+    let mut parts = v.split('-');
+    let (version, trace, span) = (parts.next()?, parts.next()?, parts.next()?);
+    if version != "00" || trace.len() != 32 || span.len() != 16 {
+        return None;
+    }
+    let sc = SpanContext::new(
+        TraceId::from_hex(trace).ok()?,
+        SpanId::from_hex(span).ok()?,
+        TraceFlags::SAMPLED,
+        true,
+        TraceState::default(),
+    );
+    sc.is_valid().then(|| opentelemetry::Context::new().with_remote_span_context(sc))
+}
+
+/// The current span as a `traceparent`, for a request to another service, so its spans join
+/// this trace. None outside a traced span.
+pub fn traceparent() -> Option<String> {
+    use opentelemetry::trace::TraceContextExt;
+    use tracing_opentelemetry::OpenTelemetrySpanExt;
+    let cx = tracing::Span::current().context();
+    let sc = cx.span().span_context().clone();
+    sc.is_valid().then(|| format!("00-{}-{}-{:02x}", sc.trace_id(), sc.span_id(), sc.trace_flags().to_u8()))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn a_requests_trace_is_the_pages_from_its_traceparent() {
+        use opentelemetry::trace::TraceContextExt;
+        let with = |v: &str| {
+            let mut h = axum::http::HeaderMap::new();
+            h.insert("traceparent", v.parse().unwrap());
+            remote_parent(&h, None)
+        };
+        let cx = with("00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01").expect("parsed");
+        assert_eq!(cx.span().span_context().trace_id().to_string(), "4bf92f3577b34da6a3ce929d0e0e4736");
+        assert!(cx.span().span_context().is_remote());
+        // Anything else from the browser starts a trace of its own.
+        for bad in ["", "00-xyz-00f067aa0ba902b7-01", "01-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01",
+                    "00-00000000000000000000000000000000-00f067aa0ba902b7-01", "00-4bf92f3577b34da6a3ce929d0e0e4736"] {
+            assert!(with(bad).is_none(), "{bad}");
+        }
+        assert!(remote_parent(&axum::http::HeaderMap::new(), None).is_none());
+        // From the query, where a beacon or Safari's player cannot send a header (#192).
+        let q = remote_parent(&axum::http::HeaderMap::new(), Some("x=1&traceparent=00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01"));
+        assert_eq!(q.unwrap().span().span_context().trace_id().to_string(), "4bf92f3577b34da6a3ce929d0e0e4736");
+        assert!(remote_parent(&axum::http::HeaderMap::new(), Some("traceparent=nonsense")).is_none());
+    }
+}