metrics: Internalize metric label sets #917

Job	Run time
Test with DPDK enabled / test	15m 38s
Test with C++20 modules enabled / test	11m 28s
Test (clang++, C++20, dev) / test	8m 7s
Test (clang++, C++20, debug) / test	10m 16s
Test (clang++, C++20, release) / test	8m 43s
Test (clang++, C++23, debug) / test	10m 43s
Test (clang++, C++23, dev) / test	8m 28s
Test (clang++, C++23, release) / test	15m 28s
Test (g++, C++20, debug) / test	11m 51s
Test (g++, C++20, release) / test	9m 22s
Test (g++, C++20, dev) / test	7m 36s
Test (g++, C++23, debug) / test	12m 4s
Test (g++, C++23, dev) / test	7m 44s
Test (g++, C++23, release) / test	9m 38s
	2h 27m 6s

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