Built-in middleware that puts static values into ctx.assigns.
Use it to stamp a procedure with fixed metadata. The handler stays unchanged.
Usage
procedure "billing.charge", &MyApp.Billing.charge/2,
middleware: [
{RpcElixir.Middleware.Assign, source: :api, environment: :prod}
]Each {key, value} in opts becomes a ctx.assigns entry. Existing keys are overwritten.