Make use of vulcand dynamic bucket config

Well, that depends on the failure policy - I would say the middleware should ignore the value if there's no header passed - as it does not know what to do with the request. But even in the case if middleware always demands the request to be set, we can use the following solution:

We can provide a static reply when I fail to read the value from the database.

My point is - I would rather allow user to specify the fallback behavior and make it a clear choice - it's either explicit rates defined via parameters or the ones specified via header.

With your existing implementation parameters `request` are really context-dependent and change their meaning based on the fact if configVar is set or not, and this is confusing.