indexer: gas metrics

[yourbuddyconner]

nomad-monitor currently has a script that calculates the gas spend for a set of agents, this is useful in forecasting future spend.

Add metrics that calculate the total cumulative spend for each of the agents.

Preferred Method: Index the right transaction data (i.e. effectiveGasPrice for the update/relay/process transactions) and calculate via DB Queries.

Alternative: Use the Etherscan provider like in gas.ts and do this in an indexing task.

Tasks:

• Ensure proper indexing of data to support this feature
• Implement gas metric (preferably a histogram)
• Extra Credit: Add API endpoint that returns a per-agent breakdown of spend.