Runtime constants can't easily be included in client

[notlesh]

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Description of bug

I have a situation where I have a constant defined in my runtime (but not in any pallet) which I want to expose with the runtime itself in some way.

Specifically, there is a constant used as a multiplier in our runtime's WeightToFee implementation. This multiplier used to be part of transaction-payment's constants until #11415 converted WeightToFee into a trait.

Before this change, we were able to query api.consts.transactionPayment.weightToFee[0] to effectively obtain this constant. This allowed a client to have all the information it needed to precisely calculate fees, which is something we used very extensively in testing.

I can think of a couple ways to address this now:

• Provide an RPC endpoint to query this
• Create a "dummy" pallet which can provide a client-side query

Both seem like they would add a lot of bloat for what seems to be a trivial problem. Am I missing anything, or would it make sense to add a feature to address this?

(Sorry, this is a cross-post from this stale stack exchange post, please forgive me!)

Steps to reproduce

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