Instead of using a global random number generator for all `randRes`,
have each value use its own. This removes the need for locking and
managing concurrent safe access to the global. Also, the field, given
the `Reservoir` type is not concurrent safe and the metric pipeline
guards this, does not need a `sync.Mutex` to guard it.
Supersedes https://github.com/open-telemetry/opentelemetry-go/pull/5815Fix#5814
### Performance Analysis
This change has approximately equivalent performance as the existing
code based on existing benchmarks.
```terminal
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/metric
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
│ old.txt │ new.txt │
│ sec/op │ sec/op vs base │
Exemplars/Int64Counter/8-8 14.00µ ± 3% 13.44µ ± 4% -3.98% (p=0.001 n=10)
│ old.txt │ new.txt │
│ B/op │ B/op vs base │
Exemplars/Int64Counter/8-8 3.791Ki ± 0% 3.791Ki ± 0% ~ (p=1.000 n=10) ¹
¹ all samples are equal
│ old.txt │ new.txt │
│ allocs/op │ allocs/op vs base │
Exemplars/Int64Counter/8-8 84.00 ± 0% 84.00 ± 0% ~ (p=1.000 n=10) ¹
¹ all samples are equal
```