Reporting telemetry to indicate how many one-time reports are using user credentials instead of API keys

[mikecote]

Once https://github.com/elastic/kibana-team/issues/1331 is implemented, reports should mainly use API keys as the authentication method. However, there are some situations where a fallback mechanism will still be needed where it uses the user credentials:

• API keys are disabled in Elasticsearch xpack.security.authc.api_key.enabled
• The Kibana Encrypted saved-objects plugin is using a generated encryption key

In those situations, the report will use the user credentials to authenticate the report. We should track how often this happens so we know when we can make a breaking change and stop supporting one-time reports in such situations.

Definition of Done

• We can get a count of times a cluster used user credentials to generate a report
• We can get a count of clusters that are still using user credentials
• Tests

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)