HTTP metrics require that names be unique

`InstrumentedHttpClientConnectionManager` requires unique names or throws an Exception :

```
java.lang.IllegalArgumentException: A metric named org.apache.http.conn.HttpClientConnectionManager.available-connections already exists
	at com.codahale.metrics.MetricRegistry.register(MetricRegistry.java:91) ~[io.dropwizard.metrics.metrics-core-3.1.0.jar:3.1.0]
	at com.codahale.metrics.httpclient.InstrumentedHttpClientConnectionManager.<init>(InstrumentedHttpClientConnectionManager.java:57) ~[io.dropwizard.metrics.metrics-httpclient-3.1.0.jar:3.1.0]
```

Since our http clients can be destroyed and re-created, that leaves us with 2 options :
- a fully unique ID per http client (e.g. with a `UUID`)
- no ID and reuse the same `InstrumentedHttpClientConnectionManager`

This PR implements the second idea.