Fix HllRedisCounter overwriting know events aggregation with symbol instead of string (backport)

Related to #414253 (closed)

master bug fix !122625 (merged)

Description from the fix:

HllRedicsCounter caches known events at class level, that mean that overwritten attribute persists between multiple metrics. Because symbols were used to overwrite aggregation attribute to weekly, and it was persisted later on in aggregated metrics calculation some events had aggregation configured with symbols and other string, causing events.all? { |event| event[:aggregation] == aggregation } to return false and raise error

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