Add Zuora client-facing errors graph for Fulfillment platform group

Related to gitlab-org/customers-gitlab-com#5579 (closed)

This MR adds a new graph to the main CustomersDot Grafana page. This graph shows the rate of customer-facing Zuora errors that have been captured over time.

Here's the Prometheus client in Production for the related counter metric: https://prometheus-gke.prdsub.gitlab.net/graph?g0.expr=customers_dot_zuora_error&g0.tab=1&g0.stacked=0&g0.show_exemplars=0&g0.range_input=1h

Here's a snapshot of this graph addition: https://dashboards.gitlab.net/dashboard/snapshot/UW8geZ8fMUTD5jIyVq2iNJe47DwGuEWT?orgId=1