Feature Request: Middleware

As discussed in #128, it might be a cool idea to have a "middleware" layer in asyncpg where people can register their own code to be ran pre or post query.

One usecase is asyncpgsa, which is a project to allow sqlalchemy objects to be passed to asyncpg connection methods. If there was a middleware layer, asyncpgsa could simply be a middleware that would run right before asyncpg prepared the query to convert the sqlalchemy object into a string. This would make asyncpgsa stupid simple, and just be a conversion layer, allowing it to get out of the way, and let asyncpg be the source of truth for everything else.

Another use case is metrics. My org is starting to track metrics per endpoint and per database query. If asyncpg had middleware support, we could simply inject a middleware that tracked the query and the time it took, and send that information into our metric platform. This could be done today by wrapping asyncpg and adding a method around it that tracks the metrics. It would be amazing if instead asyncpg could be used directly, and the middleware handles that portion for me.

Middlewares could potentially be "added" when the pool is created. (they can only exist on a pool maybe?)

Anyways, I am happy to try to implement this if you would like, but first wanted to get a general discussion going on the topic.

まず issue #128 を読み、その後、pool の作成と接続メソッドを調べて、pre-query および post-query middleware をどこで登録・呼び出しできるかを判断してください。asyncpgsa の変換とメトリクスのユースケースを比較し、実装前に必要な API、スコープ、順序、async の動作、テストを定義してください。