@Override
    public void freeEntry(BasicPoolEntry entry, boolean reusable,
            long validDuration, TimeUnit timeUnit) {
        releaseCounter.increment();
        super.freeEntry(entry, reusable, validDuration, timeUnit);
    }
A connection pool that provides Servo counters to monitor the efficiency. 
Three counters are provided: counter for getting free entries (or reusing entries),
counter for creating new entries, and counter for every connection request.

@author awang
ribbon-httpclient/src/main/java/com/netflix/http4/NamedConnectionPool.java