Fixes#17255.
The implementation happens mostly in the Add and Sub method, but the reconciliation works for all relevant operations. For example, you can now `rate` over a range wherein the custom bucket boundaries are changing.
Any custom bucket reconciliation is flagged with an info-level annotation.
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Signed-off-by: Linas Medziunas <linas.medziunas@gmail.com>
Signed-off-by: Linas Medžiūnas <linasm@users.noreply.github.com>