Allow ConnectionConfig to be provided a custom metric registry for the Hikari data source

[mccheah]

Currently, ConnectionConfig#getHikariConfig bootstraps a MetricsRegistry via SharedMetricRegistries#getOrCreate (see .here However, some users may have their own MetricRegistry that they wish to bootstrap into the config. We can use HikariConfig#setMetricRegistry after the ConnectionConfig has returned the HikariConfig, but in doing so we leave the redundant MetricRegistry hanging in SharedMetricRegistries. We should provide a means for ConnectionConfig to be provided the MetricRegistry directly when creating the HikariConfig.