Third party module integration (define hook_statsd_*)

To enable use os StatsD by third party modules (contrib or custom), the module could define
- hook_statsd_info to retrieve a list of supported metrics with their descriptions
- hook_statsd_metrics to return values for all/some of the supported metrics

This way, you would be able to add the corresponding hook to apachesolr module to report on index document counts, for instance.
Another use case like #2360055: Report queue API statistics would be easy to solve, perhaps in a statsd_queue submodule.