metrics: Fix bug in collecting metrics for /api/varz #3066

Job	Run time
lint_core	12s
Build frontend e2e test image	4m 11s
Build frontend unit test image	1m 10s
Build backend e2e test image	1m 58s
Lint mwdb-core web sources	45s
Build mwdb-core core image	1m 48s
Build mwdb-core web image	32s
Perform frontend unit tests	29s
Perform backend e2e tests	6m 22s
Perform frontend e2e tests	3m 39s
Push test images on Docker Hub	0s
Push images on Docker Hub	0s
	21m 6s

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