in src/main/java/org/apache/sling/commons/metrics/internal/MetricsServiceImpl.java [191:197]
private <T extends Metric> T register(String name, T metric) throws IllegalArgumentException {
final Metric existing = metrics.putIfAbsent(name, metric);
if (existing != null) {
throw new IllegalArgumentException("A metric named " + name + " already exists");
}
return metric;
}