Fix missing f-prefix in exponential histogram max_size error messages (#5434)

* Fix missing f-prefix in exponential histogram max_size error messages

The continuation strings of the two ValueError messages in
_ExponentialBucketHistogramAggregation lacked the f prefix, so the
literal text {self._min_max_size} / {self._max_max_size} was printed
instead of the actual limits (2 / 16384).

* Add changelog fragment

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Co-authored-by: Emídio <9735060+emdneto@users.noreply.github.com>