[EXPORTER] Prometheus: Remove explicit timestamps from metric points #8513

Job	Run time
CMake gcc 12 (maintainer mode, C++11, async)	12m 7s
CMake gcc 12 (maintainer mode, sync)	11m 11s
CMake test (without otlp-exporter)	7m 42s
CMake test (with abseil)	8m 43s
CMake clang 14 (maintainer mode, async)	16m 32s
CMake clang 14 (maintainer mode, sync)	16m 34s
CMake gcc 4.8 (with otlp exporter)	14m 32s
CMake gcc 4.8 (without otlp exporter)	5m 42s
CMake gcc 12 (maintainer mode, async)	11m 5s
CMake test (with modern protobuf,grpc and abseil)	34m 37s
CMake msvc (maintainer mode)	15m 13s
CMake test (with opentracing-shim)	7m 43s
CMake test (with otlp-exporter)	21m 9s
CMake C++20 test(Clang with libc++)	17m 5s
CMake C++20 test(GCC)	12m 12s
CMake test (without otlp-exporter and with async export)	9m 10s
Bazel asan config	10m 36s
Bazel noexcept	23m 10s
Bazel nortti	23m 8s
Bazel on MacOS	25m 52s
Bazel with async export	28m 4s
Bazel	21m 6s
Bazel tsan config	25m 1s
Bazel valgrind	27m 7s
Bazel with external abseil	20m 20s
CMake do not install test (with otlp-exporter)	25m 54s
CMake install test (with abseil)	5m 56s
CMake test (build shared libraries with otlp-exporter and static gRPC)	20m 36s
Plugin -> CMake	1m 10s
Code coverage	16m 25s
Benchmark	13m 57s
Copyright	10s
DocFX check	1m 27s
Format	17s
markdown-lint	9s
misspell	11s
CMake -> exporter proto	22m 46s
Bazel Windows	37m 31s
Plugin -> CMake Windows	13m 28s
CMake (With async export) -> exporter proto	23m 47s
CMake -> exporter proto (Build as DLL)	24m 21s
	10h 33m 46s

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