.send is not a function on FallbackProvider

[cloudonshore]

Is your feature request related to a problem? Please describe.
I'm trying to implement eth node redundancy into an API that calculates gas prices using the eth_feeHistory call. I noticed that on a fallback provider I can't send RPC calls like

provider.send('eth_feeHistory', [
      lookbackNumber,
      blockNumber,
      percentiles,
    ])

Describe the solution you'd like
I would like a way to call arbitrary RPC calls using the fallback provider, where the fallback provider attempts to contact nodes sequentially, and if it encounters a failure, it moves onto the next node.

Describe alternatives you've considered
Using try/catches everywhere in my application to try using two different JSON RPC providers to make sure I have redundancy in case one provider fails or goes offline.

[ricmoo]

You cannot do this with the FallbackProvider.

A Provider in ethers is a porcelain interface. It is not tied to JSON-RPC providers, as not all providers are JSON-RPC-based.

In v6, there may be a more generic way to execute an operation against each provider in the FallbackProvider, but in v5 this isn't possible at all.