Add docs for `metric_time` and `duration_metric_time` #21

Job	Run time
Documentation	3m 1s
Tests, Julia 1 - ubuntu-latest - x64 - pull_request	1m 37s
Tests, Julia 1 - windows-latest - x64 - pull_request	3m 2s
Tests, Julia 1 - macos-latest - x64 - pull_request	2m 51s
	10m 31s

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