Adding support for Druid post aggregations (continued)

[x4base]

Hi, this is still not so easy to use, but at least I can get the post aggregated average value using this now. It's at least another step forward.
This feature is very important to me and many others as you can see in the comments in the original pull requests ( #133 ).
Please consider merging it to the master if you think this is kind of ready. Or please give me feedbacks so I can make the whole feature work sooner.

Currently only TableViz is supported. If you think it looks good, I can add support to more other viz.
Thanks

[landscape-bot]

Repository health decreased by 0.28% when pulling b9941d4 on x4base:post_agg2 into bd47a29 on airbnb:master.

• 7 new problems were found (including 0 errors and 3 code smells).
• No problems were fixed.

[landscape-bot]

Repository health decreased by 0.10% when pulling f4e8b06 on x4base:post_agg2 into bd47a29 on airbnb:master.

• 3 new problems were found (including 0 errors and 0 code smells).
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[coveralls]

Coverage decreased (-0.2%) to 80.749% when pulling f4e8b06 on x4base:post_agg2 into bd47a29 on airbnb:master.

[landscape-bot]

Repository health decreased by 0.17% when pulling 152b47c on x4base:post_agg2 into bd47a29 on airbnb:master.

• 5 new problems were found (including 0 errors and 0 code smells).
• No problems were fixed.

[coveralls]

Coverage decreased (-0.5%) to 80.525% when pulling 152b47c on x4base:post_agg2 into bd47a29 on airbnb:master.

[x4base]

Now aggregators are treated as if they are just normal metrics

[mistercrunch]

I did a little bit more work on this locally and wanted to basically define post aggs as metrics, using the DruidMetric model as opposed to a new model. Then it's just a matter of figuring out which metrics are post aggs and generating the write query.

[x4base]

Do you mean you have another local branch in your machine that is similar to this pull request? Based on your old pull request or this request? Will you push your commit? Do I have to change anything in this PR or just wait for your commit?

[mistercrunch]

Opting for #418 instead of this PR