Add functions var, std

[mesejo]

This add the functions var and std. It mirrors the same implementation of the other metric aggregations.
The changes were made in _metric_aggs, for two reasons:

1. To keep all logic related to the aggregating functions in the same place.
2. extended_stats are metric aggregations in Elasticsearch API.

Closes #171

[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]

From a code perspective this looks good. We also need to add mad and other aggregations too.

I'm also seeing an opportunity to refactor the .mean(), .var(), etc functions to use .aggregate() so we don't have to maintain both routes of applying these aggregations.

[mesejo]

@sethmlarson Thanks for the comments, I agree with your perspective. Also I'm pretty positive there is a bug with median:

>>> df = ed.DataFrame(es, 'flights')
>>> df[['AvgTicketPrice']].agg(['median'])

throws this,

Traceback (most recent call last):
File "/home/daniel/PycharmProjects/eland/venv/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 3331, in run_code
exec(code_obj, self.user_global_ns, self.user_ns)
File "", line 1, in
df[['AvgTicketPrice']].agg(['median'])
File "/home/daniel/PycharmProjects/eland/venv/lib/python3.6/site-packages/eland-7.6.0a4-py3.6.egg/eland/dataframe.py", line 1316, in aggregate
return self._query_compiler.aggs(func)
File "/home/daniel/PycharmProjects/eland/venv/lib/python3.6/site-packages/eland-7.6.0a4-py3.6.egg/eland/query_compiler.py", line 458, in aggs
return self.operations.aggs(self, func)
File "/home/daniel/PycharmProjects/eland/venv/lib/python3.6/site-packages/eland-7.6.0a4-py3.6.egg/eland/operations.py", line 513, in aggs
response["aggregations"][es_agg[0] + "" + field][es_agg[1]]
KeyError: '50.0'

The call is made in the same way as with extended_stats (a tuple), but is a call to percentiles and do not match the Elasticsearch API.

[sethmlarson]

.median() probably needs the same fix I made to aggregate("mean")! You have to unpack an additional JSON object for the values within a quantile aggregation. Can you fix that too?

Even more reason to consolidate the implementations. :) That can be done in a separate PR though. Thanks for finding all these issues.

[mesejo]

I think is better to do it in a separate PR

[sethmlarson]

Okay, the v8.0.0 failures are unrelated. Merging this now! :) Thanks much!