check if metric is registererd

[miekg]

See #4021 We seem to be forgetting to register metrics (too often).

The exiting metrics naming test (test/metric_naming_test.go) should be extended to test for this as well - at least assuming it's easy enough to do so

[miekg]

See this thanos-io/thanos#2102
We need to fix our declaration and registration. I.e. the fix the code not the test

[chrisohaver]

I.e. the fix the code not the test

I have opted to use promauto in some other (simpler) projects.