Extend metrics to support building indexing progress indicators

[tk-o]

Problem / use case

I want to build a web UI where I'd present current indexing progress, grouped by chain ID.

For example, knowing total ponder_historical_total_blocks tells me how many blocks are there to be processed. However, I don't know if there's any metric on indexed blocks count. This would be required to do this simple progress bar math:

indexedBlocksCount / historicalBlocksCount

Proposed solution

Include a metric that tracks a count of all blocks indexed by chain ID.