fix(sdk-trace-base)!: use parent-base-always-on on incorrect input (#5494)

Author: pichlermarc

CHANGELOG.md

@@ -112,6 +112,8 @@ For semantic convention package changes, see the [semconv CHANGELOG](packages/se
   * (user-facing): all configuration previously possible via `window.OTEL_*` is now not supported anymore
     * If you have been using the `envDetector` in browser environments, please migrate to manually creating a resource.
     * Note: Node.js environment variable configuration continues to work as-is.
+* fix(sdk-trace-base)!: use `ParentBasedAlwaysOnSampler` over `AlwaysOnSampler` when bogus data is supplied to `OTEL_TRACES_SAMPLER`
+  * this aligns the SDK implementation with the specification
 * feat(core)!: drop `getEnv()`, `getEnvWithoutDefaults()` [#5481](https://github.com/open-telemetry/opentelemetry-js/pull/5481) @pichlermarc
   * (user-facing): `getEnv()` has been replaced by `getStringFromEnv()`, `getNumberFromEnv()`, `getBooleanFromEnv()`, `getStringListFromEnv()`
     * these new functions do not include defaults, please inline any defaults if necessary (example: `getStringFromEnv("OTEL_FOO") ?? "my-default"`)

packages/opentelemetry-sdk-trace-base/src/config.ts

@@ -31,7 +31,6 @@ const enum TracesSamplerValues {
   TraceIdRatio = 'traceidratio',
 }
 
-const FALLBACK_OTEL_TRACES_SAMPLER = TracesSamplerValues.AlwaysOn;
 const DEFAULT_RATIO = 1;
 
 /**
@@ -96,9 +95,11 @@ export function buildSamplerFromEnv(): Sampler {
       });
     default:
       diag.error(
-        `OTEL_TRACES_SAMPLER value "${sampler}" invalid, defaulting to "${FALLBACK_OTEL_TRACES_SAMPLER}".`
+        `OTEL_TRACES_SAMPLER value "${sampler}" invalid, defaulting to "${TracesSamplerValues.ParentBasedAlwaysOn}".`
       );
-      return new AlwaysOnSampler();
+      return new ParentBasedSampler({
+        root: new AlwaysOnSampler(),
+      });
   }
 }