Fix bug which was making sending counter metrics with hostname=None

https://github.com/DataDog/dd-agent/blob/v4.x/aggregator.py#L520-L531
This function was using the contexts stored in the context index but if
hostname was not set in the statsd call, it was storing None as a
hostname, now it will store the dogstatsd server hostname as it will be
the one which will be sent when metrics are flushed anyway.
Fix https://github.com/DataDog/dogweb/issues/4645