[exporters/prometheus] Sanitize labels according to Prometheus spec (… #1472

Job	Run time
Bazel noexcept	2m 39s
Bazel nortti	2m 57s
Bazel on MacOS	4m 51s
Bazel asan config	3m 16s
Bazel	17m 18s
Bazel with async export	7m 57s
Bazel tsan config	2m 17s
Bazel valgrind	3m 33s
Bazel with external abseil	2m 3s
Benchmark	1m 57s
CMake test (with abseil)	7m 5s
CMake clang 14 (maintainer mode, async)	17m 37s
CMake clang 14 (maintainer mode, sync)	18m 17s
CMake do not install test (with otlp-exporter)	27m 40s
CMake gcc 4.8 (with otlp exporter)	16m 58s
CMake gcc 4.8 (without otlp exporter)	7m 0s
CMake gcc 12 (maintainer mode, C++11, async)	12m 21s
CMake gcc 12 (maintainer mode, sync)	10m 25s
CMake gcc 12 (maintainer mode, async)	10m 36s
CMake install test (with abseil)	6m 29s
CMake test (with modern protobuf,grpc and abseil)	26m 50s
CMake msvc (maintainer mode)	13m 36s
CMake test (with opentracing-shim)	6m 16s
CMake test (build shared libraries with otlp-exporter and static gRPC)	28m 11s
CMake test (with otlp-exporter)	21m 11s
CMake test (without otlp-exporter)	6m 36s
CMake C++20 test(GCC)	13m 36s
CMake C++20 test(Clang with libc++)	17m 22s
CMake test (without otlp-exporter and with async export)	6m 49s
Code coverage	16m 13s
Copyright	7s
DocFX check	1m 28s
Format	16s
markdown-lint	9s
misspell	5s
Plugin -> CMake	1m 9s
CMake -> exporter proto	20m 44s
Bazel Windows	36m 19s
Plugin -> CMake Windows	12m 28s
CMake (With async export) -> exporter proto	18m 16s
CMake -> exporter proto (Build as DLL)	17m 31s
	7h 28m 28s

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