chore: import upstream snapshot with attribution

apps/sim/lib/copilot/request/go/propagation.ts

@@ -0,0 +1,57 @@
+import { type Context, context } from '@opentelemetry/api'
+import { W3CTraceContextPropagator } from '@opentelemetry/core'
+
+const propagator = new W3CTraceContextPropagator()
+const headerSetter = {
+  set(carrier: Record<string, string>, key: string, value: string) {
+    carrier[key] = value
+  },
+}
+
+const headerGetter = {
+  keys(carrier: Headers): string[] {
+    const out: string[] = []
+    carrier.forEach((_, key) => {
+      out.push(key)
+    })
+    return out
+  },
+  get(carrier: Headers, key: string): string | undefined {
+    return carrier.get(key) ?? undefined
+  },
+}
+
+/**
+ * Injects W3C trace context (traceparent, tracestate) into outbound HTTP
+ * headers so Go-side spans join the same OTel trace tree as the calling
+ * Sim span.
+ *
+ * Usage: spread the result into your fetch headers:
+ *   fetch(url, { headers: { ...myHeaders, ...traceHeaders() } })
+ */
+export function traceHeaders(
+  carrier?: Record<string, string>,
+  otelContext?: Context
+): Record<string, string> {
+  const headers: Record<string, string> = carrier ?? {}
+  propagator.inject(otelContext ?? context.active(), headers, headerSetter)
+  return headers
+}
+
+/**
+ * Extracts W3C trace context from incoming request headers (traceparent /
+ * tracestate) and returns an OTel Context seeded with the upstream span.
+ *
+ * Use this at the top of inbound Sim route handlers that Go calls into
+ * (e.g. /api/billing/update-cost, /api/copilot/api-keys/validate) so the
+ * Sim-side span becomes a proper child of the Go-side client span in the
+ * same trace — closing the round trip in Jaeger.
+ *
+ * When no traceparent is present (e.g. calls from a browser or a client
+ * that hasn't been instrumented), this returns `context.active()`
+ * unchanged, and any span started under it becomes a new root — the same
+ * behavior as before this helper existed.
+ */
+export function contextFromRequestHeaders(headers: Headers): Context {
+  return propagator.extract(context.active(), headers, headerGetter)
+}