Include metrics in connector log

The connector currently exposes metrics via JMX, but it would be nice if the metrics were also in the connector logs, since logs are more readily available for post-hoc investigations.

Add a new `couchbase.metrics.interval` connector config property. Specify as a duration, like "10m" for ten minutes. Default to 10 minutes.

If non-zero, the connector writes diagnostic metrics to the log at that interval.