fix(otel): don't throw when calling setActiveSpan at root

ext/telemetry/telemetry.ts

@@ -145,7 +145,7 @@ function hrToMs(hr: [number, number]): number {
 
 export function enterSpan(span: Span): Context | undefined {
   if (!span.isRecording()) return undefined;
-  const context = (CURRENT.get() || ROOT_CONTEXT).setValue(SPAN_KEY, span);
+  const context = (CURRENT.get() ?? ROOT_CONTEXT).setValue(SPAN_KEY, span);
   return CURRENT.enter(context);
 }
 
@@ -254,9 +254,9 @@ class Tracer {
       throw new Error("startActiveSpan requires a function argument");
     }
     if (options?.root) {
-      context = undefined;
+      context = ROOT_CONTEXT;
     } else {
-      context = context ?? CURRENT.get();
+      context = context ?? CURRENT.get() ?? ROOT_CONTEXT;
     }
     const span = this.startSpan(name, options, context);
     const ctx = CURRENT.enter(context.setValue(SPAN_KEY, span));

tests/specs/cli/otel_basic/__test__.jsonc

@@ -33,6 +33,10 @@
     "links": {
       "args": "run -A main.ts links.ts",
       "output": "links.out"
+    },
+    "start_active_span": {
+      "args": "run -A main.ts start_active_span.ts",
+      "output": "start_active_span.out"
     }
   }
 }

tests/specs/cli/otel_basic/start_active_span.out

@@ -0,0 +1,48 @@
+{
+  "spans": [
+    {
+      "traceId": "00000000000000000000000000000001",
+      "spanId": "0000000000000001",
+      "traceState": "",
+      "parentSpanId": "",
+      "flags": 1,
+      "name": "top level span",
+      "kind": 1,
+      "startTimeUnixNano": "[WILDCARD]",
+      "endTimeUnixNano": "[WILDCARD]",
+      "attributes": [],
+      "droppedAttributesCount": 0,
+      "events": [],
+      "droppedEventsCount": 0,
+      "links": [],
+      "droppedLinksCount": 0,
+      "status": {
+        "message": "",
+        "code": 0
+      }
+    },
+    {
+      "traceId": "00000000000000000000000000000002",
+      "spanId": "0000000000000002",
+      "traceState": "",
+      "parentSpanId": "",
+      "flags": 1,
+      "name": "root span",
+      "kind": 1,
+      "startTimeUnixNano": "[WILDCARD]",
+      "endTimeUnixNano": "[WILDCARD]",
+      "attributes": [],
+      "droppedAttributesCount": 0,
+      "events": [],
+      "droppedEventsCount": 0,
+      "links": [],
+      "droppedLinksCount": 0,
+      "status": {
+        "message": "",
+        "code": 0
+      }
+    }
+  ],
+  "logs": [],
+  "metrics": []
+}

tests/specs/cli/otel_basic/start_active_span.ts

@@ -0,0 +1,12 @@
+// Copyright 2018-2025 the Deno authors. MIT license.
+
+import { trace } from "npm:@opentelemetry/[email protected]";
+
+const tracer = trace.getTracer("example-tracer");
+
+tracer.startActiveSpan("top level span", (span) => {
+  span.end();
+});
+tracer.startActiveSpan("root span", { root: true }, (span) => {
+  span.end();
+});