Add context metric tags for admin handler

[vytautas-karpavicius]

What changed?
Added context metrics tags for admin handler. Context tags currently contain which transport the request is coming from (tchannel or grpc).

Why?
All other handlers have it. It was probably forgotten to add for admin handler as well.

How did you test it?
Verified on staging2

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build b92fbf7b-c4b6-498c-b9d4-0b6f779065c1

• 7 of 26 (26.92%) changed or added relevant lines in 1 file are covered.
• 48 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.01%) to 56.395%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
service/frontend/adminHandler.go	7	26	26.92%

Files with Coverage Reduction	New Missed Lines	%
common/types/shared.go	1	26.9%
service/history/queue/timer_queue_processor_base.go	1	78.56%
common/membership/rpServiceResolver.go	2	73.58%
service/history/task/transfer_active_task_executor.go	2	71.06%
common/persistence/nosql/nosqlplugin/cassandra/workflow.go	3	49.32%
common/task/fifoTaskScheduler.go	4	85.57%
common/types/mapper/thrift/shared.go	4	65.02%
common/persistence/nosql/nosqlplugin/cassandra/workflowUtils.go	12	75.24%
common/persistence/nosql/nosqlplugin/cassandra/workflowParsingUtils.go	19	75.97%

Totals
Change from base Build a9a61159-e980-446b-8089-99d7854a1386:	-0.01%
Covered Lines:	78672
Relevant Lines:	139501

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💛 - Coveralls