double numWorkers() {
            int result = 0;
            for (EventLoopGroup group : registry) {
                // Purge event loop groups that were shutdown.
                if (group.isShutdown()) {
                    registry.remove(group);
                    continue;
                }
                result += Iterators.size(group.iterator());
            }
            return result;
        }
An actual implementation of {@link EventLoopMetrics}.
core/src/main/java/com/linecorp/armeria/common/metric/EventLoopMetrics.java