ISW: diagnosis page should indicate refresh rate of metrics

## What?

It should be clear if metrics are real-time; or what period they refresh at.

## Why?

Metrics refresh periodically. This can be confusing, if you are expecting it to be real-time. Expected changes don't show up; and metric increments later with no clear relationship to the flow of log messages.

## Your Environment

* Graylog Version: 6.2