Add tombstoned metric to JMX

[Claimundefine]

Adding metric for when schemas are hard deleted. These metrics do not overlap with soft-deleted metrics.

It should be mentioned that the original ticket was to add soft-delete metric, but in fact the missing metric is hard deletes. Note that soft delete count >= hard delete count, since the schema must be soft deleted first.

Testing
Added test cases, as well as manually running requests.

FAQ:

What happens when a soft deleted schema gets restored then deleted again?

Soft deleted schemas pertaining to a subject can be retrieved with the deleted=true flag, which allows you to find the deleted schema and recreate it using the same definition. However, this creates a new version for the subject, and does not map to the old version.

What about the workaround to restore hard-deleted schemas?

It is difficult to tell when a hard-deleted schema is restored, since it is recreated using the same definition then manually mapped to a schema id. These are separate operations that cannot definitively be mapped to restoring a hard-deleted schema.

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