## Issue Addressed
Which issue # does this PR address?
## Proposed Changes
Avoids cloning the `BeaconState` each time Prometheus scrapes our metrics (generally every 5s 😱).
I think the original motivation behind this was *"don't hold the lock on the head whilst we do computation on it"*, however I think is flawed since our computation here is so small that it'll be quicker than the clone.
The primary motivation here is to maintain a small memory footprint by holding less in memory (i.e., the cloned `BeaconState`) and to avoid the fragmentation-creep that occurs when cloning the big contiguous slabs of memory in the `BeaconState`.
I also collapsed the active/slashed/withdrawn counters into a single loop to increase efficiency.
## Additional Info
NA
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