Show metrics for a published campaign.

The metrics returned in the response to this request are computed asynchronously. Check these fields for
more information:

• data.metrics.updated_at - This field indicates when the metrics were last updated. Sometimes this field
  can be null when the asynchronous worker that serves these metrics is first starting up.

• data.status - This field indicates a status that can inform the client if updated metrics can be expected.

When a request is made to the asynchronous worker that provides these metrics, the worker checks the age of
the existing metrics and if they are more than 60 seconds old a job is started to update them. Clients can expect
updated metrics soon if the data.status field has the value updating.

One option for clients that wish to show updated metrics is to schedule a request for 5 seconds in the future
when data.status has the value updating. Simply returning the current stale metrics is not expensive for
the server so it is safe to poll every 5 seconds or so to check for updates when they are expected.

When data.status has the value ready clients can schedule the next request at time data.metrics.updated_at plus
61 seconds.

When data.status has the value starting clients can schedule the next request for 5 seconds in the future
like with updating. All of the counters will be initialized with the value 0 but the starting status exists
for clients that may not wish to display the 0 values to the user.