feat: endpoint that exposes mempool size divided by tx status

[gregorycoppola]

This is a cross-post of https://github.com/blockstack/stacks-blockchain/issues/3001 There are issues to think about on both blockchain and API sides.

I did an analysis that shows that only around 16.5% of transactions in the mempool are actually "ready to process":

{
  total_examples: 785,
  count_map: Map(5) {
    'pending--nonce-correct' => 130,
    'pending--nonce-too-high' => 203,
    'pending--nonce-too-low' => 162,
    'dropped_replace_by_fee' => 289,
    'dropped_stale_garbage_collect' => 1
  }
}

code: https://github.com/gregorycoppola/stx-share/blob/main/mempool/nonces.js

A dashboard like https://stacksonchain.com/dashboards/Mempool/9 seems to use the 785 number, when the number of transactions that are actually ready to be processed is only 130. This leads users to have a worse impression of what is going on than is warranted.

We should figure out a way to display a more realistic number, so that the mempool doesn't look more full than it is.

Separately, some transactions are not processing. But, a more accurate assessment should help us address those as well.

[rafaelcr]

An API endpoint exposing these mempool stats could be very useful

[gregorycoppola]

More generally, we should have some dashes that are public like https://stacksdata.info/#mempool

I think we have some private dashboards, but we should consider making more public dashboards available on the explorer, because users like looking at these.

[zone117x]

This issue (and issue https://github.com/hirosystems/stacks-blockchain-api/issues/984 to some degree) seem useful but not yet clear on what role the API should play in generating and exposing this data.

Moved into the icebox for now.