Instrument the logger with error count metrics by level

The ability to do sanity checks against logged errors and warning counts could be helpful for several reasons. One of the most obvious would be as a way to verify that no errors were logged during a (semi-) automated upgrade or restart process.

Fortunately, this is easy to enable as described here: https://dropwizard.github.io/metrics/3.1.0/manual/logback/

It was pointed out by jjordan that this ability should exist in current version if the user is willing to drop in the right jars and modify the appender config.

It would also be helpful as a programmatic feature with a toggle to enable or disable, possibly with a cassandra.yaml config parameter. There may be some users who would prefer to disable it to avoid calling another appender. If testing shows the overhead for this to be sufficiently low, we could just leave it on by default.

These should be exposed via JMX when they are enabled.