Uw update or create

Added

*  `exometer:update_or_create(Name, Value)` and,
* `exometer:update_or_create(Name, Value, Type, Options)`

The former tries to update a metric and, if it doesn't exist, creates it from a default template, and updates it.
The latter, tries to update and, if it doesn't exist, calls `exometer:ensure(Name, Type, Opts)` to either verify that the metric exists and is of type `Type`, or creates the metric, and updates it.