[c++] Lightly refactor MapMetric, fix small bug, and add unit tests

[pgrinspan]

Contributes to #7021

• Simplify CalMapAtK a tad
• Ignore queries with no positive labels
• Unify weighted/unweighted Eval paths via GetQueryWeight helper
• Add test_map_metric.cpp with tests for CalMapAtK and Eval

[jameslamb]

Thanks very much!

In the future, please do link to previous conversations when a PR is related to one (I've added a link to #7021 in the description). This helps us connect related conversations in the project.

@metpavel when you have time could you please review this?

[jameslamb]

By the way @pgrinspan it looks like you local git configuration is not set up to tie your commits to your GitHub account. See #7143 (comment) for details on how to fix that.

[pgrinspan]

By the way @pgrinspan it looks like you local git configuration is not set up to tie your commits to your GitHub account. See #7143 (comment) for details on how to fix that.

Ok, I've applied it.
I'm guessing it's not retroactive but hopefully not a big issue? Let me know if it is, and what to do about it - thanks!

[jameslamb]

Thanks, recent commit looks good!

It doesn't apply retroactively. You could do that by force-pushing over those previous commits but don't bother....when this is merged we'll squash everything into a single commit and it'll be attributed to your GitHub account.

Just noting because:

1. it might cause issues for you in other GitHub projects
2. it might mean that your next PR will incorrectly be labeled as coming from a "new contributor" and again require us to manually run the automated tests

Thanks for taking the time to contribute, we greatly appreciate it 😊