[query] Fix graphite summarize/summarize to add missing agg functions

[yyin-sc]

What this PR does / why we need it:

Fixes graphite summarize/summarize to add missing agg functions, including

1. median
2. diff
3. range
4. stddev
5. Multiply
6. count

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[codecov]

Codecov Report

Merging #3583 (0bbc8db) into master (c35fa2d) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3583   +/-   ##
======================================
  Coverage    55.9%   55.9%           
======================================
  Files         551     551           
  Lines       61832   61832           
======================================
+ Hits        34578   34598   +20     
+ Misses      24134   24122   -12     
+ Partials     3120    3112    -8     

Flag	Coverage Δ
aggregator	57.2% <ø> (ø)
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.4% <ø> (+<0.1%)	⬆️
m3em	46.4% <ø> (ø)
metrics	19.7% <ø> (ø)
msg	74.1% <ø> (-0.2%)	⬇️

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[robskillington]

Probably can remove all this too:

// specificationFunc determines the output series specification given a series list.
type specificationFunc func(ts.SeriesList) string
func sumSpecificationFunc(series ts.SeriesList) string {
	return wrapPathExpr("sumSeries", series)
}
func averageSpecificationFunc(series ts.SeriesList) string {
	return wrapPathExpr("averageSeries", series)
}

[robskillington]

LGTM