Currently we have a metric which counts the number of available signing keys: https://github.com/onflow/flow-evm-gateway/blob/main/services/requester/single_tx_pool.go#L163C15-L163C35 .

However, this doesn't give us a clear view of how many transactions were dropped due to no signing keys being available. This error occurs at https://github.com/onflow/flow-evm-gateway/blob/main/services/requester/keystore/key_store.go#L60, and it is only returned as part of the eth_sendRawTransaction JSON-RPC endpoint.

We should add a metric to count the number of dropped transactions.