fix(python): add fallback in case content-length isn't available

[kanadgupta]

🧰 What's being changed?

When creating a test app with https://github.com/readmeio/flask-metrics-example, I ran into this error:

Traceback (most recent call last):
  File "~/flask-starter/venv/lib/python3.7/site-packages/readme_metrics/MetricsMiddleware.py", line 68, in __call__
    content_length = int(environ["CONTENT_LENGTH"])
KeyError: 'CONTENT_LENGTH'

Per PEP 3333 definition of CONTENT_LENGTH:

The contents of any Content-Length fields in the HTTP request. May be empty or absent.

TODO

• Tests

🧪 Testing

Set up https://github.com/readmeio/flask-metrics-example and observe the error.