Creating and using the key is the way to go.
The usage is free until your application reaches 25.000 calls per day on 90 consecutive days.
BTW.: In the google Developer documentation it says you shall add the api key as option {key:yourKey} when calling the API to create new instances.
This however doesn’t shush the console warning.
You have to add the key as a parameter when including the api.
<script src="https://maps.googleapis.com/maps/api/js?key=yourKEYhere"></script>
Get the key here: GoogleApiKey Generation site