adjust live spam metrics inputs (txs_sent)

**Is your feature request related to a problem? Please describe.**

When transactions are sent, the metrics collected by `print_progress_report` only reflect what's eventually confirmed on chain and saved to the DB. This gives the averaging algorithm a skewed input, which makes the metrics inaccurate, especially in short-duration spam runs.

**Describe the solution you'd like**

We need a new way of getting `txs_sent` -- it's currently defined as `confirmed_txs + failed_txs` which is derived from the DB (`run_txs`).
We need to internally track the number of txs sent as we send them, and pass a thread-safe (read-only) reference to that number to `print_progress_report`.

**Additional context**

see [`contender_core::util::print_progress_report`](https://github.com/flashbots/contender/blob/main/crates/core/src/util.rs#L140)