bugfix: cannot get response size from body attr for streamed responses

[edaniszewski]

This PR:

• fixes a bug in the metrics collection where streamed responses will raise an exception due to the streamed response not having a body attribute

timestamp='2019-11-07T19:25:23.740522Z' level='debug' event='processing request' method='GET' ip='10.48.29.113' path='/v3/readcache' params={'start': ['2019-11-07T19:25:18Z']}
Exception occurred in one of response middleware handlers
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/sanic/app.py", line 983, in handle_request
    request, response
  File "/usr/local/lib/python3.6/site-packages/sanic/app.py", line 1271, in _run_response_middleware
    _response = await _response
  File "/usr/local/lib/python3.6/site-packages/synse_server/metrics.py", line 118, in before_response
    if response.body is not None:
AttributeError: 'StreamingHTTPResponse' object has no attribute 'body'

[KylerBurke]

👍