Investigate Counter collection performance

[dashpole]

#7427 greatly improved the performance of Measure() for sums, at the expense of Collect. That makes sense given Measure is expected to be called much more often. But it would be good to double-check if there are ways to mitigate the degredation, particularly in terms of allocations.

See https://github.com/open-telemetry/opentelemetry-go/actions/runs/18314631364/job/52151483097

This will likely apply to other optization PRs as well.