[exporters/prometheus] Remove explicit timestamps from metric points … #8520

Job	Run time
CMake test (without otlp-exporter)	7m 7s
CMake clang 14 (maintainer mode, async)	17m 27s
CMake clang 14 (maintainer mode, sync)	17m 6s
CMake gcc 12 (maintainer mode, async)	13m 22s
CMake gcc 12 (maintainer mode, C++11, async)	10m 28s
CMake gcc 12 (maintainer mode, sync)	10m 51s
CMake msvc (maintainer mode)	11m 34s
CMake test (with abseil)	6m 14s
CMake do not install test (with otlp-exporter)	21m 19s
CMake gcc 4.8 (with otlp exporter)	16m 45s
CMake gcc 4.8 (without otlp exporter)	7m 52s
CMake test (with modern protobuf,grpc and abseil)	27m 27s
CMake test (with opentracing-shim)	6m 0s
CMake test (build shared libraries with otlp-exporter and static gRPC)	24m 19s
CMake C++20 test(Clang with libc++)	13m 51s
CMake test (with otlp-exporter)	19m 51s
CMake C++20 test(GCC)	13m 5s
CMake test (without otlp-exporter and with async export)	9m 10s
Bazel noexcept	19m 41s
Bazel nortti	19m 1s
Bazel	19m 37s
Bazel with async export	19m 6s
Bazel valgrind	27m 30s
Bazel with external abseil	25m 41s
CMake install test (with abseil)	6m 24s
Plugin -> CMake	1m 28s
Bazel asan config	10m 34s
Bazel on MacOS	28m 8s
Bazel tsan config	19m 30s
Benchmark	12m 34s
Code coverage	13m 52s
Copyright	6s
Format	25s
markdown-lint	7s
misspell	5s
CMake -> exporter proto	21m 28s
Bazel Windows	35m 26s
Plugin -> CMake Windows	9m 30s
CMake (With async export) -> exporter proto	21m 4s
CMake -> exporter proto (Build as DLL)	19m 28s
DocFX check	1m 13s
	9h 45m 46s

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