Add new background job to collect billing metrics for each tenant and
send them to the HTTP endpoint.
Metrics are cached, so we don't send non-changed metrics.
Add metric collection config parameters:
metric_collection_endpoint (default None, i.e. disabled)
metric_collection_interval (default 60s)
Add test_metric_collection.py to test metric collection
and sending to the mocked HTTP endpoint.
Use port distributor in metric_collection test
review fixes: only update cache after metrics were send successfully, simplify code
disable metric collection if metric_collection_endpoint is not provided in config
4235f97c6a