doc: document MetricRegistry pointer safety guarantees

Add explicit documentation to Find, FindCaseInsensitive, FindWithAliases,
List, and ListByCategory clarifying that returned *Metric pointers are
safe to hold across concurrent Unregister calls.

Metric objects are immutable after registration: Register panics on
duplicate names (no in-place mutation), Unregister only deletes entries
from the map (never modifies the Metric struct), and no mutator methods
exist on the Metric type. The sync.RWMutex protects the map structures,
while the immutability invariant protects the returned pointers.

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