Summary: "person", "origin_visit", and "skipped_content" objects are not counted by `in_memory.Storage.stat_counters()`
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Context:
All `*_add` methods add full object content in a dedicated structure (eg. `origin_add`/`origin_add_one` adds an object in `self._origins`).
Additionally, most of them also add a "pointer" to that object in a common structure stored as `self._objects`, so they can be retrieved by `object_find_by_sha1_git`.
Not all of them do, because they manipulate objects that don't have a `sha1`, so we did not implement them because it wouldn't make sense.
However, `self._objects` is also used by `stat_counters()`; so counters about these objects are always 0.
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What needs to be changed:
This issue resides entirely in `swh-storage/swh/storage/in_memory.py`.
To fix this issue, you need to add code to various `*_add` methods so they also add entries to `self._objects`, which will be counted by `in_memory.Storage.stat_counters()`