docs: add docs for RateLimiter

---
Update docs/user_guide/ratelimiter.md
---
docs: add upstream_request_filter to fix forbidden error of RateLimiter
---
Update ratelimiter.md

update rate_limiter_map initialize code

Co-authored-by: Dany Gagnon <ddanygagnon@gmail.com>
Co-authored-by: Xia Lei <xialeistudio@gmail.com>
Includes-commit: 461d384966
Includes-commit: 80231711f9
Includes-commit: 99ed87f309
Includes-commit: bffc9f4f23
Replicated-from: https://github.com/cloudflare/pingora/pull/120

.bleep

@@ -1 +1 @@
-4c6da000f956f3c13473dee0c6302ac0126418dd
+9ec558778457c28b1ffbb1d7b4f4f5ab370b97a5

docs/user_guide/index.md

@@ -20,6 +20,7 @@ In this guide, we will cover the most used features, operations and settings of
 * [Examples: take control of the request](modify_filter.md)
 * [Connection pooling and reuse](pooling.md)
 * [Handling failures and failover](failover.md)
+* [RateLimiter quickstart](ratelimiter.md)
 
 ## Advanced topics (WIP)
 * [Pingora internals](internals.md)

docs/user_guide/ratelimiter.md

@@ -0,0 +1,116 @@
+# **RateLimiter quickstart**
+Pingora provides a crate `pingora-limits` which provides a simple and easy to use rate limiter for your application.
+
+## Steps
+1. Add the following dependencies to your `Cargo.toml`:
+   ```toml
+   pingora-limits = "0.1.0"
+   lazy_static = "1.4.0"
+   ```
+2. Declare a global rate limiter map to store the rate limiter for each client. In this example, we use `appid`.
+3. Override the `request_filter` method in the `ProxyHttp` trait to implement rate limiting.
+   1. Retrieve the client appid from header.
+   2. Retrieve the current window requests from the rate limiter map. If there is no rate limiter for the client, create a new one and insert it into the map.
+   3. If the current window requests exceed the limit, return 429 and set RateLimiter associated headers.
+   4. If the request is not rate limited, return `Ok(false)` to continue the request.
+
+## Example
+```rust
+use std::collections::HashMap;
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
+use async_trait::async_trait;
+use lazy_static::lazy_static;
+use pingora::http::ResponseHeader;
+use pingora::prelude::*;
+use pingora_limits::rate::Rate;
+
+fn main() {
+   let mut server = Server::new(Some(Opt::default())).unwrap();
+   server.bootstrap();
+   let mut upstreams = LoadBalancer::try_from_iter(["1.1.1.1:443", "1.0.0.1:443"]).unwrap();
+   // Set health check
+   let hc = TcpHealthCheck::new();
+   upstreams.set_health_check(hc);
+   upstreams.health_check_frequency = Some(Duration::from_secs(1));
+   // Set background service
+   let background = background_service("health check", upstreams);
+   let upstreams = background.task();
+   // Set load balancer
+   let mut lb = http_proxy_service(&server.configuration, LB(upstreams));
+   lb.add_tcp("0.0.0.0:6188");
+
+   // let rate = Rate
+   server.add_service(background);
+   server.add_service(lb);
+   server.run_forever();
+}
+
+pub struct LB(Arc<LoadBalancer<RoundRobin>>);
+
+impl LB {
+   pub fn get_request_appid(&self, session: &mut Session) -> Option<String> {
+      match session.req_header().headers.get("appid").map(|v| v.to_str()) {
+         None => None,
+         Some(v) => match v {
+            Ok(v) => Some(v.to_string()),
+            Err(_) => None
+         }
+      }
+   }
+}
+
+
+// global limiter
+lazy_static! {
+    static ref RATE_LIMITER_MAP: Arc<Mutex<HashMap<String, Rate>>> = {
+       Arc::new(Mutex::new(HashMap::new()))
+    };
+}
+// max request per second per client
+static MAX_REQ_PER_SEC: isize = 1;
+
+#[async_trait]
+impl ProxyHttp for LB {
+   type CTX = ();
+
+   fn new_ctx(&self) -> Self::CTX {
+      ()
+   }
+   async fn upstream_peer(&self, _session: &mut Session, _ctx: &mut Self::CTX) -> Result<Box<HttpPeer>> {
+      let upstream = self.0
+              .select(b"", 256)
+              .unwrap();
+      // Set SNI
+      let peer = Box::new(HttpPeer::new(upstream, true, "one.one.one.one".to_string()));
+      Ok(peer)
+   }
+   async fn upstream_request_filter(&self, _session: &mut Session, upstream_request: &mut RequestHeader, _ctx: &mut Self::CTX) -> Result<()> where Self::CTX: Send + Sync {
+      upstream_request.insert_header("Host", "one.one.one.one").unwrap();
+      Ok(())
+   }
+   async fn request_filter(&self, session: &mut Session, _ctx: &mut Self::CTX) -> Result<bool> where Self::CTX: Send + Sync {
+      let appid = match self.get_request_appid(session) {
+         None => return Ok(false), // no client appid found, skip rate limiting
+         Some(addr) => addr
+      };
+
+      // retrieve the current window requests
+      let curr_window_requests = {
+         let mut rate_limiter_map = RATE_LIMITER_MAP.lock().unwrap();
+         let rate_limiter = rate_limiter_map.entry(appid.clone()).insert_or_with(|| Rate::new(Duration::from_secs(1)));
+         rate_limiter.observe(&appid, 1)
+      };
+      if curr_window_requests > MAX_REQ_PER_SEC { // rate limited, return 429
+         let mut header = ResponseHeader::build(429, None).unwrap();
+         header.insert_header("X-Rate-Limit-Limit", MAX_REQ_PER_SEC.to_string()).unwrap();
+         header.insert_header("X-Rate-Limit-Remaining", "0").unwrap();
+         header.insert_header("X-Rate-Limit-Reset", "1").unwrap();
+         session.set_keepalive(None);
+         session.write_response_header(Box::new(header)).await?;
+         return Ok(true);
+      }
+      Ok(false)
+   }
+}
+```