Address valid review comments

Signed-off-by: Eric Peterson <ericpeterson@spotify.com>

packages/backend-defaults/src/alpha/entrypoints/tracing/DefaultTracingService.ts

@@ -89,6 +89,13 @@ export class DefaultTracingService implements TracingService {
 
   readonly context: TracingServiceContextAPI = {
     active: () => fromOtelContext(otelContext.active()),
+    // `otelContext.with` is synchronous: it activates `ctx`, invokes `fn`,
+    // then restores the previous active context before this call returns.
+    // When `fn` is async, the AsyncLocalStorage context manager installed
+    // by the OTel SDK is what keeps `ctx` active across the callback's
+    // `await`s. If no context manager is registered (e.g. in a test that
+    // does not wire up the OTel SDK) the `await` continuations will run
+    // outside `ctx`.
     with: async <T>(
       ctx: TracingServiceContext,
       fn: () => T | Promise<T>,

packages/backend-test-utils/src/alpha/services/TracingServiceMock.ts

@@ -130,6 +130,12 @@ export interface MockedTracingServicePropagationAPI
  * `mockReturnValue` / `mockImplementation`, which takes precedence over
  * the default behaviour.
  *
+ * Unlike the real `DefaultTracingService`, the mock's `startActiveSpan`
+ * does **not** resolve `options.credentials` from `options.request` via
+ * `httpAuth`. Tests that need principal-derived span attributes should
+ * supply `options.credentials` directly on the span options, or assert
+ * on the raw `options` captured by `startActiveSpan.mock.calls`.
+ *
  * @alpha
  */
 export interface TracingServiceMock extends TracingService {