MGS: SpPoller missed tick behavior should probably not be `Burst`

[hawkw]

@jgallagher points out to me that the SpPoller in MGS' SP metrics module uses a tokio::time::interval with the default MissedTickBehavior, which is Burst. This means that if we miss a one-second polling interval because, say, MGS was busy servicing other requests, or due to the vagaries of host OS scheduling, we'll send a burst of SP metrics requests right after each other. This is probably not the ideal behavior here, since the goal is just to do one poll every second. I should probably change this.