fix(prometheus): stabilize label maps in MetricVec.GetMetricWith

[immanuwell]

fixes #1951

cc @vesari

GetMetricWith(Labels) could hash one label set, then store another if the same map gets mutated while the metric is being created. that can bubble up as collected before with the same name and label values, kind of nasty.

This stabilizes the map input first, then reuses the label values path. WithLabelValues(...) stays as is.

Repro:

1. check out the parent of this commit
2. run go test ./prometheus -run TestMetricVecWithLabelsKeepsLabelValuesStable -count=1
3. it fails with collected metric "test" ... was collected before with the same name and label values
4. run the same command on this branch
5. it passes

Checks:

• go test ./...
• cd exp && go test ./...
• go test -race ./prometheus -run 'TestMetricVecWithLabelsKeepsLabelValuesStable|TestMetricVecWithConstraints|TestCurryVec|TestCurryVecWithConstraints|TestDelete|TestDeleteLabelValues|TestDeletePartialMatch' -count=1

[bwplotka]

Thanks but we prefer performance vs making it ultra safe for all cases. However some commentary would be useful. See #1951 (comment)

Feel free to push back, we can always reopen 🤗