Add a worker time-spent-idle counter to the ruler

[leth]

The blocked_workers metric can't give us a good indication of how busy the ruler is, because work happens every 15s and we sample every ~15s.
The evalLatency metric indicates when evaluations are late, i.e. when we have reached/exceeded our capacity, but doesn't indicate how close we are to it in normal circumstances.

Using an idle time counter should give us an idea of how much slack we have in the queue processing; if it approaches 0, then we know our workers are at capacity.

Since counters are monotonic, this should allow us to measure utilisation without hitting the same sampling issue.