Assert metrics tags in UTs

[nachocano]

Describe the bug
We should assert the metrics tags in UTs

Expected behavior
Metrics tags are asserted and validated in UTs

Additional context
From PR review #1864 (review)

[nachocano]

Actually this should be done in the StatsReporter object, which knows about the underlying tags that are sent. We are already doing that. E.g.,

eventing/pkg/broker/ingress/stats_reporter_test.go

Line 53 in 3794bdb

wantTags := map[string]string{

Closing this bug.
fyi @Harwayne