[exporters/prometheus] Several refactors #8526

Job	Run time
CMake gcc 12 (maintainer mode, C++11, async)	10m 41s
CMake gcc 12 (maintainer mode, sync)	13m 28s
CMake clang 14 (maintainer mode, sync)	16m 13s
CMake test (without otlp-exporter)	8m 39s
CMake test (with abseil)	6m 38s
CMake clang 14 (maintainer mode, async)	16m 56s
CMake gcc 12 (maintainer mode, async)	14m 39s
CMake gcc 4.8 (with otlp exporter)	15m 10s
CMake gcc 4.8 (without otlp exporter)	7m 0s
CMake msvc (maintainer mode)	11m 51s
CMake test (with opentracing-shim)	8m 52s
CMake C++20 test(GCC)	17m 29s
CMake test (without otlp-exporter and with async export)	6m 47s
Bazel noexcept	18m 43s
Bazel nortti	24m 26s
Bazel	23m 34s
Bazel with async export	19m 25s
Bazel valgrind	27m 50s
Bazel with external abseil	18m 56s
CMake do not install test (with otlp-exporter)	25m 32s
CMake test (with modern protobuf,grpc and abseil)	27m 58s
CMake test (build shared libraries with otlp-exporter and static gRPC)	19m 27s
CMake install test (with abseil)	6m 20s
CMake test (with otlp-exporter)	24m 47s
CMake C++20 test(Clang with libc++)	11m 7s
Plugin -> CMake	1m 10s
Bazel asan config	13m 7s
Bazel on MacOS	22m 39s
Bazel tsan config	20m 39s
Benchmark	13m 4s
Code coverage	16m 12s
Copyright	11s
DocFX check	37s
Format	22s
markdown-lint	10s
misspell	6s
CMake -> exporter proto	17m 13s
Bazel Windows	34m 40s
Plugin -> CMake Windows	12m 1s
CMake (With async export) -> exporter proto	17m 18s
CMake -> exporter proto (Build as DLL)	17m 36s
	9h 49m 33s

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