fix: prevent slice aliasing in LabelValues.With

[Yanhu007]

Fixes #1296

Problem

LabelValues.With uses append directly, which reuses the underlying array when spare capacity exists. This causes sibling With calls to overwrite each other's label values:

base := counter.With("a", "1", "b", "2")
c1 := base.With("c", "3", "d", "4")
c2 := base.With("e", "5")
// c2 overwrites c1's backing array: c1 now has ["a","1","b","2","e","5","d","4"]

This affects all metrics types (Counter, Gauge, Histogram) across all providers that use LabelValues.With.

Fix

Allocate a new slice in With to prevent aliasing the parent's backing array:

result := make(LabelValues, len(lvs), len(lvs)+len(labelValues))
copy(result, lvs)
return append(result, labelValues...)

Added a test that verifies sibling With calls don't corrupt each other.