Update exemplar random seleection

[tsloughter]

Can see better performance for simple exemplars when finding what the next seen number that will result in an insertion will be before hand and when a new exemplar is offered we do nothing but increment the seen count.

See the Go code for more explanation: https://github.com/open-telemetry/opentelemetry-go/blob/main/sdk/metric/internal/exemplar/rand.go