Ensure safe access to global rand.Rand

Update rand.Rand usage to add locking around a global rand.Rand
instance, which is not safe for concurrent access by multiple
goroutines.

Fixes open-telemetry#5814.

sdk/metric/internal/exemplar/rand.go

@@ -123,7 +123,9 @@ func (r *randRes) Offer(ctx context.Context, t time.Time, n Value, a []attribute
 	} else {
 		if r.count == r.next {
 			// Overwrite a random existing measurement with the one offered.
+			rngMu.Lock()
 			idx := int(rng.Int63n(int64(cap(r.store))))
+			rngMu.Unlock()
 			r.store[idx] = newMeasurement(ctx, t, n, a)
 			r.advance()
 		}