Changed error logging to warnings for unknown OTEL_NODE_RESOURCE_DETECTORS values as per issue #4882.

I referenced the OpenTelemetry specification document, which clarified that diag.warn should be used for unrecognized values that don't critically break the system.

Author: RichardChukwu

experimental/packages/opentelemetry-sdk-node/src/utils.ts

@@ -54,7 +54,7 @@ export function getResourceDetectorsFromEnv(): Array<DetectorSync> {
   return resourceDetectorsFromEnv.flatMap(detector => {
     const resourceDetector = resourceDetectors.get(detector);
     if (!resourceDetector) {
-      diag.error(
+      diag.warn(
         `Invalid resource detector "${detector}" specified in the environment variable OTEL_NODE_RESOURCE_DETECTORS`
       );
     }