What metrics can influence or help with such an estimation?
I am mostly interested in applying such metrics on GitHub projects, using their issues history. Based on how fast they solved their previous issues, or other GitHub data, what metrics can I compute to help me with this estimation?
Note that I'm not interested in any automatic effort estimation methods. Only metrics that can, potentially, help with that. I will use those metrics to come up with my own automatic effort estimation methods, but that is beyond the scope of this question.
If you have any suggestions for a GitHub-specific metric using their search+filters or their API, that is also great.
The metrics should be simple. By simple, I mean that they should return some numeric values, where higher (or lower) means a more (or less) active project, relative to whatever baseline.
What I'm actually needing this for
I am working on a machine learning project that is supposed to estimate effort in hours based on metrics. For this, I am interested in using one for activity. I want something that is easy to compute using github data and that I can reference somehow. I want to focus on easy computation rather than quality, in hopes that other metrics and data I will use will fill in the quality part. I'm also happy to use one of the metrics I came up with, as long as there is some reference that they have been, at one point, used in either research or practice, with at least some degree of success.