message: fix(metrics): Check for cls entry sources
  The LayoutShift.sources field may not exist in certain browsers, even if
  the LayoutShift API is there. This PR patches logic that adds CLS data
  to make sure that `._clsEntry.sources` exists.
  Ref:
diff: @@ -214,7 +214,8 @@ export class MetricsInstrumentation {
  transaction.setTag('lcp.size', this._lcpEntry.size);
  }
  - if (this._clsEntry) {
  + // See: https://developer.mozilla.org/en-US/docs/Web/API/LayoutShift
  + if (this._clsEntry && this._clsEntry.sources) {
  logger.log('[Measurements] Adding CLS Data');
  this._clsEntry.sources.map((source, index) =>
  transaction.setTag(`cls.source.${index + 1}`, htmlTreeAsString(source.node)),
  