`om health`: Add `--json` #1457

Job	Run time
main (x86_64-linux, false)	3m 39s
main (aarch64-darwin, false)	8m 12s
github-actions-runners (ubuntu-latest)	2m 25s
website	0s
static-binary-check (ubuntu-latest)	3s
github-actions-runners (macos-latest)	4m 55s
static-binary-check (macos-latest)	4s
	19m 18s

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