Agents: use configured `nativeToken.decimals` for balance metrics

[tkporter]

Problem

At the moment, we assume that a particular protocol type will always have the same number of decimals for their native token: https://github.com/hyperlane-xyz/hyperlane-monorepo/blob/e737d998ff23b5aa6cf2be9634789ca5dc5a4275/rust/hyperlane-core/src/metrics/agent.rs#L24-L29

This is not the case for Cosmos - atm our COSMOS_DECIMALS is set to 6, which is correct for Neutron, but Injective's decimal count is 18: https://github.com/cosmos/chain-registry/blob/9a9a5860aebc27b280f4377adca93eab82c59e78/injective/assetlist.json#L14

Iiuc this led to us misinterpreting injective balance metrics in alerts / dashboards

Solution

We already have the nativeToken config that comes from the SDK chain metadatas (at least for EVM chains - recall that atm all non-EVM chain configs are handcrafted. This should be addressed in https://github.com/hyperlane-xyz/hyperlane-monorepo/issues/3325).

• We can start reading this config in, for example: https://github.com/hyperlane-xyz/hyperlane-monorepo/blob/e737d998ff23b5aa6cf2be9634789ca5dc5a4275/rust/config/mainnet3_config.json#L23-L27
• The configured native token decimals should be used for our balance metrics -- we should consider still keeping protocol-wide defaults to keep required configs slim
• Specify the nativeToken in the JSON configs for any chains that don't already have them - e.g. all non-EVM chains are likely to not have this. As Injective is the motivating case for this ticket, also add injective
• Add this config change to the agent docs

[yorhodes]

note that the chainMetadata.nativeToken definitions are being moved out of the SDK to the registry