Add num-running-compaction-iterators statistic #4035

Job	Run time
build-linux	16m 36s
check-format-and-targets	25s
build-linux-cmake-with-folly-coroutines	30m 42s
build-linux-cmake-with-folly	26m 59s
build-linux-cmake-with-benchmark	18m 58s
build-linux-encrypted_env-no_compression	15m 24s
build-linux-make-with-folly	27m 36s
build-linux-make-with-folly-lite-no-test	6m 1s
build-linux-release	13m 47s
build-examples	10m 16s
build-fuzzers	8m 37s
build-linux-clang10-asan	16m 58s
build-linux-clang10-ubsan	17m 3s
build-linux-clang10-clang-analyze	17m 51s
build-linux-clang13-mini-tsan	21m 52s
build-linux-clang-13-no_test_run	7m 29s
build-linux-clang-no_test_run	12m 22s
build-linux-cmake-mingw	11m 47s
build-linux-cmake-with-folly-lite-no-test	7m 42s
build-linux-gcc-10-cxx20-no_test_run	9m 28s
build-linux-gcc-11-no_test_run	10m 16s
build-linux-gcc-8-no_test_run	8m 38s
build-linux-java	14m 24s
build-linux-mini-crashtest	28m 38s
build-linux-release-rtti	12m 0s
build-linux-static_lib-alt_namespace-status_checked	19m 41s
build-linux-unity-and-headers	12m 43s
build-macos	23m 54s
build-macos-cmake (true)	43m 27s
build-windows-vs2019	56m 22s
build-macos-cmake (false)	53m 1s
build-linux-arm	14m 50s
build-linux-java-pmd	11m 34s
build-linux-java-static	11m 17s
build-macos-java	25m 40s
build-macos-java-static	24m 4s
build-macos-java-static-universal	30m 29s
	11h 38m 51s

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