This page will show you, the provided (starting with version 4.0) Rest API to create/read/update/delete metadata values.

Be aware that:

• most of the calls required a logged in user session. (Further details regarding authentication: https://developer.atlassian.com/jiradev/jira-apis/about-the-jira-rest-apis/jira-rest-api-tutorials/jira-rest-api-example-cookie-based-authentication )
• the user interface displays a maximum of 500 metadata values for a project/component/version/user/group, but with the rest interface you can manage more than 500.