refactor(layers/prometheus): provide builder APIs #1659

Job	Run time
check_docs	3m 17s
build_all_features	4m 42s
build_default_features (ubuntu-latest)	37s
build_all_platforms (ubuntu-latest)	2m 34s
build_under_wasm	44s
build_default_features (macos-latest)	36s
build_all_platforms (macos-latest)	2m 3s
check_clippy	3m 31s
build_default_features (windows-latest)	2m 24s
build_all_platforms (windows-latest)	5m 42s
unit	6m 14s
check_msrv	31s
	32m 55s

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