Migrate project_imports metrics to instrumentation classes

What does this MR do and why?

Related to #351486 (closed).

This migrates project_imports metrics in Gitlab::UsageData to instrumentation classes.

Usually, this means that the metric can be removed from Gitlab::UsageData, because it will be picked up using the metadata in the metric definitions. However, for now they need to be left in Gitlab::UsageData using the add_metric method, for the total to be calculated.

In the next iteration the total will be moved to be calculated in a separate instrumentation class, which will execute a redundant query since instrumentation class values can't rely on results of other metrics, but it will reduce complexity nonetheless.

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