refactor(layers/prometheus): provide builder APIs #1654

Job	Run time
build_all_features	4m 45s
build_default_features (ubuntu-latest)	35s
check_clippy	3m 35s
build_default_features (macos-latest)	47s
check_docs	2m 58s
build_default_features (windows-latest)	1m 59s
check_msrv	31s
build_under_wasm	42s
unit	4m 34s
build_all_platforms (ubuntu-latest)	2m 32s
build_all_platforms (macos-latest)	2m 0s
build_all_platforms (windows-latest)	6m 8s
	31m 6s

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