foo_bucket{le="1"} should be turned into foo_bucket{le=~"1(.0)?"}
foo_bucket{le=~"1|3"} should be turned into foo_bucket{le=~"1|3(.0)?"}

    - action: foo
      regex: foo_bucket;(1|3|5|15.5)
      sourceLabels:
      - __name__
      - le

should be adjusted

    - action: foo
      regex: foo_bucket;(1|3|5|15.5)(\.0)?
      sourceLabels:
      - __name__
      - le

Aggregation by the `le` and `quantile` labels for vectors that contain the old and
new formatting will lead to unexpected results, and range vectors that span the
transition between the different formatting will contain additional series.
The most common use case for both is the quantile calculation via
`histogram_quantile`, e.g.
`histogram_quantile(0.95, sum by (le) (rate(histogram_bucket[10m])))`.
The `histogram_quantile` function already tries to mitigate the effects to some
extent, but there will be inaccuracies, in particular for shorter ranges that
cover only a few samples.