tps prometheus

[bernardoaraujor]

The work done so far was mostly following polkadot-introspector as a reference for writing the minimum necessary boilerplate for Prometheus integration into the tps crate.

Some ToDos:

• Hasn't been tested against a real Prometheus instance. So far I only got this to compile.
• I wonder if we could use some Histogram to observe the successive block_tx_count and block_time across time.

[bredamatt]

Definitely in favour of the Histogram approach for the block_time metrics (relay and para), but it is not critical as polkadot-introspector already has these metrics. We should focus explicitly on sound metrics for TPS in this case as this is the purpose of sTPS.