test: First pass at unregistering metrics so that we can run multiple tests

[paul1r]

What this PR does / why we need it:
When running tests with a -count= value greater than 1, there are multiple places in the code that yield the following error:

panic: duplicate metrics collector registration attempted [recovered]
	panic: duplicate metrics collector registration attempted

goroutine 205 [running]:
testing.tRunner.func1.2({0x1058e1b00, 0xc00090e800})
	/opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:1631 +0x2c8
testing.tRunner.func1()
	/opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:1634 +0x47c
panic({0x1058e1b00?, 0xc00090e800?})
	/opt/homebrew/Cellar/go/1.22.2/libexec/src/runtime/panic.go:770 +0x124
github.com/prometheus/client_golang/prometheus.(*Registry).MustRegister(0x106d22380, {0xc000a02fc0, 0x1, 0x0?})
	/Users/progers/dev/src/github.com/grafana/loki/vendor/github.com/prometheus/client_golang/prometheus/registry.go:405 +0xa8
github.com/prometheus/client_golang/prometheus.MustRegister(...)
	/Users/progers/dev/src/github.com/grafana/loki/vendor/github.com/prometheus/client_golang/prometheus/registry.go:177
github.com/grafana/loki/v3/pkg/storage/chunk/client/azure.NewBlobStorageMetrics()

This PR is to ensure the metrics are unregistered so that subsequent tests can be run

Which issue(s) this PR fixes:
Relates to #8586

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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