Add one snowplow event to Usage Ping

Once we have some events being sent to the local snowplow collector (#231539 (closed)), we should we should aggregate/report one event via Usage Ping.

This will allow us to see Product Analytics + Usage Ping working end-to-end. We can verify that the data reported for this event matches what we see in Sisense.

For the purpose of this issue, the event can be aggregated and added to the counts and counts_monthly sections of the usage ping payload. In the future, we may wish to create a new JSON object for snowplow events.

/cc @jeromezng