text: Title: Scanning BLE devices returns  empty metadata
  Body: * bleak version: 0.10.0
  * Python version: Python 3.6.13  Anaconda, Inc.
  * Operating System: Windows 10 Pro
  
  ### Description
  
  Hi,
  
  I wanted to test scanning devices and print the details of the scanned device (name, metadata, etc.).
  
  The devices are detected (name, address), nevertheless the metadata field metadata = {'uuids': [], ...} does not contain the service uuids which is expected to be a list of one element, in my case 'uuids': ['680c21d9-c946-4c1f-9c11-baa1c21329e7'].
  
  The reason is I want to filter the scan output, application-side, on the device's service uuid to only report devices of interest.
  Also i checked the issue 437 and Fix KeyError: 'delegate' in CoreBluetooth backend commit.  I am using current libraries (updated ones after the issue 437). Am i doing something wrong?
  
  
  ### What I Did
  The python code:
  ```python
  async def run():
      scanner = BleakScanner()
      scanner.register_detection_callback(detection_callback)
      await scanner.start()
      await asyncio.sleep(sleep_time)
      await scanner.stop()
      scanned_devices = scanner.discovered_devices
      for d in scanned_devices:
          if d.address in Dict:
              print(d)
              print(d.metadata.values())
  ```
  ```
  The output of this code:
  E3:EB:F8:F2:9E:9D: Nordic_UART
  dict_values([[], {89: b''}])
  ```
  