Amberflo-metering-python

- Changed the `customer_id` and `customer_info` to be mandatory fields.

- Dropped support of Python versions before 3.5.
- Changed the `utc_time_millis` to be a mandatory field.

- Changed user/password authentication to an app_key.
- Small changes to the meter api:
- Change `tenant` to `customer_name` arg.
- Added a `customer_id` arg.
- Added a `unique_id` arg.
- `dimensions` is now a simple string to string map.
- Improved the meter api args validations logic.
- Removed the gzip app from the client api.
- Safer logging, and more robust client (handle exceptions better).
- Moved a few "responsibilities" to their own classes (logging, validation, meter message creation).
- Documented code and added unit tests.

- Initial release