Make "event_source" configurable when saving to the events table

We're going to have two "versions" of swatch-metrics running. Today, we're explicitly setting the "event_source" value to "prometheus" when creating MetricEvent objects.

Reference: https://github.com/RedHatInsights/rhsm-subscriptions/blob/6fce78a963b9b0f64d6c60b5ec72f8644583a36a/src/main/java/org/candlepin/subscriptions/metering/MeteringEventFactory.java#L161

This needs to be configurable so we can use the value "rhelemeter" when events are getting created by the swatch-metrics instance that is pulling from rhelemeter.

Done

1. "event_source" is configured as an env var, set in the clowdapp file, and the configured value saves in the "events" table appropriately

Testing/Verification
Use internal API against each pod, but will have to stand up a mock for rhelemeter response because we don’t have a product configured that would get its metrics (system_cpu_logical_count) from rhelemeter, and there’s not data readily available in stage rhelemeter yet.