`output_status_retry_wait` means the elapsed time from the first retry

* From https://github.com/fluent/fluentd/issues/4233.

The document says `output_status_retry_wait` means current retry_wait computed from last retry time and next retry time.

https://github.com/fluent/fluent-plugin-prometheus/blob/41fa2df366ceef7b46de154859c852926cba48a9/README.md#L122-L123

However, it actually means the elapsed time from the first retry.
We need to fix the value or the document.

https://github.com/fluent/fluent-plugin-prometheus/blob/41fa2df366ceef7b46de154859c852926cba48a9/lib/fluent/plugin/in_prometheus_output_monitor.rb#L194-L207

It is calculated as `next_time` - `start`.
`start` is the time of the first retry, so this value means the elapsed time from the first retry.

* https://github.com/fluent/fluentd/blob/aaa40f71758eb5fa7b3a89084e69a00c0910fb3b/lib/fluent/plugin/in_monitor_agent.rb#L368-L384
* https://github.com/fluent/fluentd/blob/aaa40f71758eb5fa7b3a89084e69a00c0910fb3b/lib/fluent/plugin_helper/retry_state.rb