Add a metric to track usage of the new Export Payload button

What does this MR do and why?

Follow up to https://gitlab.com/gitlab-org/product-intelligence/-/issues/502. Closes https://gitlab.com/gitlab-org/product-intelligence/-/issues/505

We added new button "Export Payload", we need to track it's usage.

Add a new metric to usage data.

How To Test

1. Visit admin/application_settings/service_usage_data
2. Click on Download Payload button

Gitlab::UsageData.data[:counts].filter {|e| e.to_s.match? /service_usage/}
=> {:service_usage_data_download_payload=>1}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #505 (closed)