refactor(layers/prometheus): provide builder APIs #1662

Job	Run time
build_all_features	5m 10s
build_all_platforms (ubuntu-latest)	2m 42s
build_all_platforms (macos-latest)	2m 21s
build_default_features (ubuntu-latest)	1m 28s
build_all_platforms (windows-latest)	7m 21s
build_default_features (macos-latest)	45s
build_default_features (windows-latest)	1m 44s
check_clippy	3m 38s
check_docs	3m 14s
check_msrv	41s
build_under_wasm	46s
unit	6m 18s
	36m 8s

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