Interacting with Metrc's API requires certification. You'll need API keys to use the library.
Metrc is the Cannabis Tracking System (CTS) used in most States where recreational cannabis is legal (and even in some that still only offer Medical Marijuana). Metrc is built and maintained by Franwell Inc., and is only accessible to licensees, verified integrators and other authorized entities.
While Metrc includes a Web based interface for licensess to interact with, it also includes an API to allow software vendors to programmatically update Metrc data. The API does not necassarily provide all of the functionality of the Web based interface, but is the only way 3rd parties can report data into Metrc.
Every software vendor in the Cannabis space is currently writing their own code for accessing and interacting with the Metrc API. This library is not intended to be a production ready replacement for writing your own code (though you're welcome to use it). It provides a partial implementation to highlight some of the peculiarities others have encountered, hopefully saving some of your sanity in the process.
For example, when you POST a new record, the API returns a status code of 200 . . . and that's it. Since you'll likely need to determine the "Id" of the record you just created, an empty response payload is less than optimal. So, this library wraps each POST with a corresponding GET, and logic to extract the newly introduced record. Likewise, the GET endpoints force you to page through data based on the "LastModified" field, and only allow a short time span (beginning 4/1/2018 requests are limited to no more than a 24 hour period). This library provides a way to make multiple calls in a single method call (though be careful . . . call limits and throttling rules are always subject to change and could cause issues).