Function processing_limit 

pub async fn processing_limit(
    __arg0: State<&AppState>,
    req: Request,
    next: Next,
) -> Result<impl IntoResponse, Error>

Middleware that implements concurrent request processing limits

This middleware:

1. Tracks number of concurrent requests being processed
2. Enforces a maximum limit on concurrent requests
3. Returns 429 Too Many Requests when limit is exceeded
4. Properly decrements counter when request processing completes

Arguments

• State(state) - Application state containing limit configuration
• req - The incoming request
• next - The next middleware in the chain