refactor(layers/prometheus): provide builder APIs #1658

Job	Run time
build_all_features	4m 48s
build_all_platforms (ubuntu-latest)	2m 39s
build_default_features (ubuntu-latest)	39s
build_all_platforms (macos-latest)	2m 26s
build_default_features (macos-latest)	46s
build_all_platforms (windows-latest)	6m 1s
build_under_wasm	44s
build_default_features (windows-latest)	1m 15s
check_clippy	3m 46s
check_docs	3m 1s
check_msrv	31s
unit	6m 30s
	33m 6s

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