Feature Request - optionally use fallback if primary runners are busy

[mikehardy]

Following integration of this action (with PR #28 merged) in, I have a feature request I'd like

Currently I believe the fallback will only be chosen if the primary is !online, which makes sense in a cost-conscious environment where you want to use a cost-avoiding self-hosted server whenever possible, even if it means jobs queue

However, my use case is throughput-conscious and cost-insensitive as our fallback are the public runners on a public repo. I'm just using self-hosted runners to add capacity to our CI queue since the public macos runners have limited concurrency.

So what I'd like to see is for the fallback to be used if the primary is busy, not just offline, thus giving our macos CI queues the combined power of my self-hosted runners and the public ones.

Any interest in that? I may be able to code it up and propose a PR if so

(API docs indicate 'busy' is a property of the API response, so this should be feasible...)

[mikehardy]

I initially did this as a boolean where any online + free primary would be sufficient

Then I realized I had a case where normally one free primary was sufficient but on a scheduled run it required 10 at once, and self-hosted runners would be insufficient then

So I redid it with a "primaries-required" count you can configure, and it will use fallback if there are not at least that many primaries free