[VSA] Issue-focused value stream
Problem to solve
Value Stream Analytics currently measures issues in some stages (like Issue and Plan), and measures merge requests in other stages (Code and Staging). That is causing confusion among users, and is arguably inaccurate. For example, if there is 1 issue that required 3 merge requests that each require 1 day, the Code stage currently says the median time is 1 day. Users are confused why it isn't 3 days. This feature would be more consistent, and less confusing, if it used issues as its only unit of measurement.
A related problem is that VSA currently assumes a 1:1 relationship between issues and MRs. That needs to be changed so VSA can understand issues with multiple MRs.
Proposal
In an issue-focused value stream, Value Stream Analytics should only measure issues. For example, the Code stage should measure "time from the first commit to the final merge, regardless of how many merge requests that spans".