Usage stats for Token service and API key service

[bizybot]

Similar to the usage stats that are reported for authentication realms, we should think of adding the support for reporting usage stats for the Token service and API key service.

[elasticmachine]

Pinging @elastic/es-security (:Security/Authentication)

[bytebilly]

We discussed in the team meeting, and even if this is a good point, we don't see high priority in adding this usage stat at the moment.

If we do that in the future, we will likely report counters rather than if services are enabled or not, as they will be on by default with TLS on the HTTP layer. The number of API keys configured in the system would bring some additional value.