Fix unbounded recursion in Metric impl for CoreMetric (metriken v0.3.5) #393

Job	Run time
rustfmt	6s
check-ubuntu-latest	9s
audit	9s
check-macos-latest	19s
clippy	17s
check-windows-latest	1m 1s
build-ubuntu-latest-release	24s
build-ubuntu-latest-debug	21s
build-macos-latest-release	39s
build-macos-latest-debug	41s
build-windows-latest-release	3m 26s
build-windows-latest-debug	1m 3s
clippy-upload	12s
verify all tests pass	1s
	8m 48s

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