@Override
    public String toString() {
        final StringBuilder builder = new StringBuilder();
        builder.append("[activeConnections=").append(activeConnections)
                .append(", scheduledConnections=").append(scheduledConnections)
                .append(", successfulConnections=").append(successfulConnections)
                .append(", failedConnections=").append(failedConnections)
                .append(", requests=").append(requests)
                .append(", tasks=").append(tasks)
                .append("]");
        return builder.toString();
    }
Collection of different counters used to gather metrics for {@link FutureRequestExecutionService}.
httpclient5/src/main/java/org/apache/hc/client5/http/impl/classic/FutureRequestExecutionMetrics.java