Fix point-in-time Prometheus metrics.

[danielmai]

Fixes #4532.

Changed metrics:

• dgraph_pending_queries_total
• dgraph_active_mutations_total
• dgraph_alpha_health_status
• dgraph_pending_proposals_total

The metrics for pending queries and active mutations would report "1" or "-1" because the metrics view was set to LastValue. This change fixes this by changing the view to a Sum so measurements of "1" and "-1" would accumulate the metrics as expected.

There were also metrics whose values were separated by tags when they should be treated as a single metric. This made metrics that were incremented and decremented show up as seperate metrics. For these metrics, the tags are excluded from the view so the metrics are shown correctly:

Example

Before:

dgraph_pending_queries_total{method="Server.Query",status=""} 100
dgraph_pending_queries_total{method="Server.Query",status="ok"} -100

After:

dgraph_pending_queries_total 0

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This change is 

[martinmr]

Fix the build issue but otherwise it

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @danielmai and @manishrjain)

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edgraph/server.go, line 805 at r1 (raw file):

	}

	isGraphQL, _ := ctx.Value(isGraphQL).(bool)

Build is failing with: ../edgraph/server.go:805:15: no new variables on left side of :=

[danielmai]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @manishrjain and @martinmr)

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edgraph/server.go, line 805 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

Build is failing with: ../edgraph/server.go:805:15: no new variables on left side of :=

Done.