Add state cloneCount metric

[dapplion]

Is your feature request related to a problem? Please describe.

New SSZ version uses a strategy of state cache transfer which provides great performance improvements. However, after cloning a state, subsequent clones will get an empty cache that must be re-populated.

Then it's important to know how many times a single state is cloned now, to estimate the effectiveness of this cache transfer strategy.

From #3760

Describe the solution you'd like

• Add a metric to get the number of clone() calls at a single state object. Every time when calling clone() add 1 to a counter of clonedCount, and register a histogram metric of cloneCount at cloned(). If states are used exactly once then the average will be 0. If states are clone multiple times the average will be greater than zero