refactor(layers/prometheus): provide builder APIs #1652

Job	Run time
build_all_features	1m 33s
build_all_platforms (ubuntu-latest)	1m 33s
build_all_platforms (macos-latest)	1m 34s
build_default_features (ubuntu-latest)	42s
build_all_platforms (windows-latest)	1m 26s
build_under_wasm	42s
build_default_features (macos-latest)	53s
build_default_features (windows-latest)	1m 17s
check_clippy	1m 32s
check_docs	1m 32s
check_msrv	34s
unit	1m 20s
	14m 38s

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