Add application-specific JMX metrics

Problem Definition

Unlike Jira, Confluence, and Bitbucket, Bamboo doesn't have application-specific JMX metrics that report plan and job stats, branches, etc...
This makes monitoring Bamboo usage and performance difficult and the experience compared to other Atlassian products is inconsistent.

Suggested Solution

Expose application-specific JMX metrics to be used by monitoring tools and produce dashboards like the following:

Workaround

Query the database or REST API for stats. Or, monitor the application logs files.