Implement DataFrameGroupBy.count()

[V1NAY8]

Closes #289

Implemented

• ed_df.groupby([...]).count()
• ed_df.groupby([...]).agg(['count'])
• ed_df.groupby([...]).agg(['max', 'min', 'count', 'mean']) maintains order
• Fixed bug where following raises exception ed_df.agg['count']
• Added tests for all the above functionality
• Add hacktoberfest-accepted label

@sethmlarson Please Review :)

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[sethmlarson]

Jenkins test this please

[sethmlarson]

Woo, this is great :) Some comments for you!

[sethmlarson]

Everything except the core functionality looks good, I'm going to pull locally and try things out to better review.

[sethmlarson]

Pulled it locally and I think I found a way to work around the count issue by adding a fake result to each bucket that gets unpacked properly in _unpack_metric_aggs(). I've attached it, you can apply it via: git apply eland-groupby-count.txt

eland-groupby-count.txt

(It's not a .patch file because apparently GitHub doesn't allow embedding those to comments?)

[V1NAY8]

Wait.. What ?? We could do it that simple, Without additional logic for maintaining order`. That was brilliant. 🤯
I guess now everything is optimized..

[sethmlarson]

This looks good to me, thanks for prettying it up! :)

[sethmlarson]

jenkins test this please

[V1NAY8]

Builds looking good :)

[sethmlarson]

Yeah went to grab coffee! ☕ Thank you for this PR :)