Support logging internal metrics from `seqcli forwarder` to a secondary Seq instance

Forwarder can be tricky to manage: in particular, it's hard to tell when a forwarder instance has gone offline.

We should:

 * ~Add `diagnostics.internalLogServerUri` and an API key setting that will turn on diagnostic logging to Seq~ done!
 * Periodically log metrics showing ingest rate, forwarding rate, and various errors
