Do not assume 'content-encoding' header is set.

[danielballan]

We are seeing a whole lot of

File "/opt/venv/lib/python3.11/site-packages/tiled/server/app.py", line 879, in capture_metrics_prometheus
    metrics.capture_request_metrics(request, response)
  File "/opt/venv/lib/python3.11/site-packages/tiled/server/metrics.py", line 128, in capture_request_metrics
    encoding = response.headers["content-encoding"]
               ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
File "/opt/venv/lib/python3.11/site-packages/starlette/datastructures.py", line 565, in __getitem__
    raise KeyError(key)
KeyError: 'content-encoding'

in the production logs.

[padraic-shafer]

Now handles missing "content-encoding"