Allow sieste address to be used as a unique identifier

Sieste now has an address endpoint for retrieving meta data from just an address, so it's not required to have all the metadata in the metric string. 

Added a whole heap of metric_id/meta helpers to get just the ID or the meta string for a metric. For any other backend, this is just a return function, but for sieste, it's functionality required. 

Note: both full metric strings and just the address are accepted in the the String part of `<Origin>~<String>` to uniquely identify a metric.