@UnstableApi
    static ConnectionPoolListener metricCollecting(MeterRegistry registry) {
        return metricCollecting(registry, new MeterIdPrefix("armeria.client"));
    }
Returns a new {@link ConnectionPoolListener} that collects metrics into the specified
{@link MeterRegistry}. The returned listener will generate the following metrics:
<table>
<caption>metrics that will be generated by this class</caption>
<tr>
  <th>metric name</th>
  <th>description</th>
</tr>
<tr>
  <td>{@code armeria.client.connections#count{state="opened"}}</td>
  <td>The number of opened connection.</td>
</tr>
<tr>
  <td>{@code armeria.client.connections#count{state="closed"}}</td>
  <td>The number of closed connections.</td>
</tr>
<tr>
  <td>{@code armeria.client.active.connections#value}</td>
  <td>The number of active connections.</td>
</tr>
</table>
core/src/main/java/com/linecorp/armeria/client/ConnectionPoolListener.java