Fix how query stats are calculated.

[martinmr]

• Add stat for how long it takes to retrieve to get a timestamp from
  zero.
• Simplify encoding_ns logic to not depend on the rest of the stats as
  that might lead to bugs and is overall pretty brittle.
• Include time to process type and schema queries in the processing
  time.

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This change is 

[martinmr]

This depends on dgraph-io/dgo#74. Once that's merged I will properly vendor the changes into Dgraph. For now I just copied the protobuf file to allow my changes to compile.

[martinmr]

Related to #3690

[manishrjain]

I haven't looked too deeply, but looks sorta alright from a high level. @gitlw can do a deeper review.

Reviewed 4 of 4 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @golangcibot)

[gitlw]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @golangcibot and @martinmr)

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query/query.go, line 2783 at r1 (raw file):

		}
	}
	req.Latency.Processing += time.Since(schemaProcessingStart)

Right now, the Latency.Processing includes two parts, the one inside ProcessQuery, and this new one you are adding here. Maybe consolidate them into one that starts the timer at the beginning of this function.

[martinmr]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @gitlw and @golangcibot)

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query/query.go, line 2783 at r1 (raw file):

Previously, gitlw (Lucas Wang) wrote…

Right now, the Latency.Processing includes two parts, the one inside ProcessQuery, and this new one you are adding here. Maybe consolidate them into one that starts the timer at the beginning of this function.

That wouldn't work, the parsing time is calculated inside ProcessQuery and then the processing time is started.

[gitlw]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @gitlw and @golangcibot)