Add maxkeys and keyspace size as metrics for dynamic samplers

[puckpuck]

Is your feature request related to a problem? Please describe.
An improperly configured set of fields used to create the keyspace used by the dynamic sampler can be difficult to diagnose.

Describe the solution you'd like
Have dynamic samplers report the MaxKeys configuration property and current keyspace size. Since multiple dynamic samplers can be used, each one would need a unique name to be identified. For samplers configured directly on a dataset/environment then, <dataset> should be used. For samplers configured as part of a Rules-based configuration the name should be <dataset>-<rule-index>-<rule-name>

[kentquirk]

Additional request for both keyspace cardinality (number of items in the keyspace) as well keyspace size in bytes.