docs: move ExEx book examples (#11616)

Co-authored-by: Alexey Shekhirin <a.shekhirin@gmail.com>

book/sources/exex/tracking-state/src/bin/2.rs

@@ -0,0 +1,73 @@
+use std::{
+    future::Future,
+    pin::Pin,
+    task::{ready, Context, Poll},
+};
+
+use alloy_primitives::BlockNumber;
+use futures_util::{FutureExt, TryStreamExt};
+use reth::api::FullNodeComponents;
+use reth_exex::{ExExContext, ExExEvent};
+use reth_node_ethereum::EthereumNode;
+use reth_tracing::tracing::info;
+
+struct MyExEx<Node: FullNodeComponents> {
+    ctx: ExExContext<Node>,
+    /// First block that was committed since the start of the ExEx.
+    first_block: Option<BlockNumber>,
+    /// Total number of transactions committed.
+    transactions: u64,
+}
+
+impl<Node: FullNodeComponents> MyExEx<Node> {
+    fn new(ctx: ExExContext<Node>) -> Self {
+        Self { ctx, first_block: None, transactions: 0 }
+    }
+}
+
+impl<Node: FullNodeComponents> Future for MyExEx<Node> {
+    type Output = eyre::Result<()>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.get_mut();
+
+        while let Some(notification) = ready!(this.ctx.notifications.try_next().poll_unpin(cx))? {
+            if let Some(reverted_chain) = notification.reverted_chain() {
+                this.transactions = this.transactions.saturating_sub(
+                    reverted_chain.blocks_iter().map(|b| b.body.transactions.len() as u64).sum(),
+                );
+            }
+
+            if let Some(committed_chain) = notification.committed_chain() {
+                this.first_block.get_or_insert(committed_chain.first().number);
+
+                this.transactions += committed_chain
+                    .blocks_iter()
+                    .map(|b| b.body.transactions.len() as u64)
+                    .sum::<u64>();
+
+                this.ctx
+                    .events
+                    .send(ExExEvent::FinishedHeight(committed_chain.tip().num_hash()))?;
+            }
+
+            if let Some(first_block) = this.first_block {
+                info!(%first_block, transactions = %this.transactions, "Total number of transactions");
+            }
+        }
+
+        Poll::Ready(Ok(()))
+    }
+}
+
+fn main() -> eyre::Result<()> {
+    reth::cli::Cli::parse_args().run(|builder, _| async move {
+        let handle = builder
+            .node(EthereumNode::default())
+            .install_exex("my-exex", |ctx| async move { Ok(MyExEx::new(ctx)) })
+            .launch()
+            .await?;
+
+        handle.wait_for_node_exit().await
+    })
+}