Monitor incoming trades

[Abrynos]

Checklist

• [x] I read and understood the Contributing Guidelines.
• [x] This is not a duplicate of an existing merge request.
• [x] I believe this falls into the scope of the project and should be part of the built-in functionality.
• [x] My code follows the code style of this project.
• [x] I have added tests to cover my changes, wherever they are necessary.
• [x] All new and existing tests pass.

Changes

As requested on our community Discord server.

New functionality

Metrics endpoint now provides information about trades received since last ASF restart.

Changed functionality

None

Removed functionality

None

Additional info

I do not want to include too much information (e.g. which items [marketable, rarity, appid, type, assetid, classid, contextid] were traded) due to performance/memory considerations. As soon as we start adding those details, this will quickly get out of hand.

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Thank you for considering the inclusion of this merge request.

[Abrynos]

Draft, as I am not completely sure about this feature. Waiting for feedback before marking as ready for review.

[Abrynos]

The little feedback I've received so far was positive so I guess if no one else is going to review, we can merge.

[JustArchi]

Thanks! :trophy: