Fix Service Ping every metric definition test

What does this MR do and why?

Related to #354134 (closed)

This MR is meant to serve two purposes:

1. Use BuildPayloadService instead of ServicePingReport because payload from ServicePingReport is later filtered by BuildPayloadService before being reported
2. Add a new test assuring that there are no unused instrumentation classes

I also fixed a typo in a comment of lib/gitlab/usage/service_ping/instrumented_payload.rb.

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How to set up and validate locally

1. Add a new Gitlab::Usage::Metrics::Instrumentations class
2. Run the spec added in this MR. It should fail now.
3. Use the new metrics class when building service ping payload, eg invoke add_metric('YourMetricClassName') in a UsageData method.
4. Run the spec added in this MR. It should pass now.

The same steps should also work for a metrics class nested inside a module, eg Gitlab::Usage::Metrics::Instrumentations::TestModule1::TestModule2::TestMetric.

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