Configurable Metrics prefix

[EdgeCaseBerg]

Hi! I saw issue 615 and figure'd it would be something pretty easy to do in a morning.
All the tests still pass, and I've written the update so that it wouldn't break anyone's metrics
that are relying on the hardcoded "pool" being added to the pool name as the base of the
metrics name.

[coveralls]

Coverage decreased (-0.3%) to 75.125% when pulling 11055d1 on EdgeCaseBerg:Metrics-Prefix into 39aa95e on brettwooldridge:dev.

[jnehlmeier]

Seems to look good.

With Metrics 4.x in the future HikariConfig would also need an API to provide metric tags because in Metrics 4.x (currently their master branch) their metrics name will be represented by an instance of MetricName (https://github.com/dropwizard/metrics/blob/master/metrics-core/src/main/java/io/dropwizard/metrics/MetricName.java) which is immutable. This class supports attaching custom tags to a metric so that metric names do not have to be relatively long just because tags are encoded directly in the metric name like I described in my issue.

So with Metrics 4.x you could do:

Map<String, String> tags = ....;
tags.put("customer", "example.com");
tags.put("host", "cluster-host-01");
tags.put("jvm-instance", "1");

// with "metricPrefix" just being, e.g. "connection-pool" or "hikaricp".
registry.register(new MetricName(metricPrefix + ".activeConnections", tags), someGauge);