cumulus/metrics: Measure the time of including a tx in a backed block

[lexnv]

This PR introduces a metric in Cumulus to measure the time it took a transaction received on the RPC layer to be included a backed block.

• A new TransactionMonitorHandle is exposed from the transaction RPC V2 API to listen on monitoring events emitted by substrate
• The monitoring event contains the block of the transaction with the timestamp associated
• A cumulus task is added to group the backed events from the relay chain with the monitoring events exposed by the RPC layer
• The spawning of RPC spawn_tasks now returns the additional TransactionMonitorHandle
• The metric is exposed under the following histogram: parachain_transaction_backed_duration

Testing Done

• Started a small network with 2 validators and 2 collators (charlie and dave -- using small_network.toml as base)
• Used subxt with a minimal working code to send a transaction to charlie
• Used the RPC-V2 API (ie ChainHeadBackend)

use std::sync::Arc;
use subxt::{
    OnlineClient, PolkadotConfig,
    backend::{chain_head::ChainHeadBackend, rpc::RpcClient},
};
use subxt_signer::sr25519::dev;

#[subxt::subxt(runtime_metadata_insecure_url = "ws://127.0.0.1:44919")]
pub mod polkadot {}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let rpc = RpcClient::from_url("ws://127.0.0.1:44919".to_string()).await?;
    let backend = ChainHeadBackend::builder().build_with_background_driver(rpc.clone());
    let api: OnlineClient<PolkadotConfig> = OnlineClient::from_backend(Arc::new(backend)).await?;

    let from = dev::charlie();
    let dest = dev::dave().public_key().into();
    let tx = Box::new(polkadot::tx().balances().transfer_allow_death(dest, 10_000));
    api.tx()
        .sign_and_submit_then_watch_default(&tx, &from)
        .await?
        .wait_for_finalized_success()
        .await?;

    Ok(())
}

• After the transaction gets finalized:

parachain_transaction_backed_duration_sum{chain="asset-hub-rococo-local"} 13.097583666
parachain_transaction_backed_duration_count{chain="asset-hub-rococo-local"} 1

Closes #8383

cc @paritytech/sdk-node

[lexnv]

/cmd prdoc --audience node_dev,node_operator --bump patch

[lexnv]

/cmd prdoc --audience node_dev --bump patch

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/15775302503
Failed job name: build-templates-node

[sandreim]

Getting it in a backed block is half way, and it is useful. Can we also add a metric to measure the transaction time to finality? This is for example what Mythical is measuring as latency.

[lexnv]

Getting it in a backed block is half way, and it is useful. Can we also add a metric to measure the transaction time to finality? This is for example what Mythical is measuring as latency.

The tx time to transition into different states (including finality) should be solved by:

• tx/metrics: Add metrics for the RPC v2 transactionWatch_v1_submitAndWatch #8345