Stats : Add time in index throttle to stats.

This PR adds the time spent throttling indexing to a single thread to the stats API.

Closes #7861
honestly this seems just wrong. The throttle is never null. It's designed to be used all the time and it's the actual lock that is used inside `class IndexThrottle` that make the difference. I think this should be implemented by using a `TimedInternalLock extends InternalLock` wrapper that is used inside `IndexThrottle` this implementation actually makes no sense to me to be honest.