[feature request] Implement fill(mean)

[pdf]

Tracking issue for implementation of fill(mean), this is the default interpolation used in some other time series DBs, and pretty important.

[beckettsean]

@pdf can you point to the documentation for this feature in another TSDB? We're not quite sure what mean would be in the absence of data. Trailing average of some sort?

[pdf]

Here's what OpenTSDB does: http://opentsdb.net/docs/build/html/user_guide/query/aggregators.html#interpolation

It's necessary to do rate calculation shorter than your sample period.

[pdf]

I think the PR for this does the right thing now, but it doesn't appear to solve the problem I wanted to. I think derivative may be calculated before the fill is performed?

[desa]

Called fill(linear)