function addFetchInstrumentationHandler(handler) {
  const type = 'fetch';
  _handlers.addHandler(type, handler);
  _handlers.maybeInstrument(type, instrumentFetch);
}
Add an instrumentation handler for when a fetch request happens.
The handler function is called once when the request starts and once when it ends,
which can be identified by checking if it has an `endTimestamp`.

Use at your own risk, this might break without changelog notice, only used internally.
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