Add prometheus metric for canary/bench

[longquanzheng]

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[coveralls]

Pull Request Test Coverage Report for Build 43de64e1-e17e-4799-8ec2-520d662a7dc6

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 53 unchanged lines in 9 files lost coverage.
• Overall coverage increased (+0.05%) to 56.859%

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Files with Coverage Reduction	New Missed Lines	%
client/history/client.go	2	39.63%
client/history/metricClient.go	2	45.94%
service/history/handler.go	2	48.42%
service/matching/matcher.go	2	91.46%
service/matching/taskListManager.go	2	74.66%
service/frontend/workflowHandler.go	4	58.59%
service/history/historyEngine.go	9	70.7%
common/persistence/nosql/nosqlplugin/cassandra/tasks.go	14	72.8%
common/persistence/nosql/nosqlTaskStore.go	16	58.37%

Totals
Change from base Build 8aee505e-f28c-4560-9262-ef26146e2dc5:	0.05%
Covered Lines:	82317
Relevant Lines:	144773

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💛 - Coveralls