Update Prometheus metrics in doc

[johnugeorge]

Update Prometheus metric names in doc. These have to be moved to Kubeflow/website separately.

Fixes: #1805

/assign @tenzen-y

[google-oss-prow]

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This pull-request has been approved by: johnugeorge

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[tenzen-y]

Thanks!
/lgtm

[coveralls]

Pull Request Test Coverage Report for Build 5772288736

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 42.754%

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Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob.go	1	91.06%

Totals
Change from base Build 5750051689:	-0.01%
Covered Lines:	3735
Relevant Lines:	8736

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💛 - Coveralls