Delayed RPC Send Using Tokens (#5923)

closes https://github.com/sigp/lighthouse/issues/5785


  The diagram below shows the differences in how the receiver (responder) behaves before and after this PR. The following sentences will detail the changes.

```mermaid
flowchart TD

subgraph "*** After ***"
Start2([START]) --> AA[Receive request]
AA --> COND1{Is there already an active request <br> with the same protocol?}
COND1 --> |Yes| CC[Send error response]
CC --> End2([END])
%% COND1 --> |No| COND2{Request is too large?}
%% COND2 --> |Yes| CC
COND1 --> |No| DD[Process request]
DD --> EE{Rate limit reached?}
EE --> |Yes| FF[Wait until tokens are regenerated]
FF --> EE
EE --> |No| GG[Send response]
GG --> End2
end

subgraph "*** Before ***"
Start([START]) --> A[Receive request]
A --> B{Rate limit reached <br> or <br> request is too large?}
B -->|Yes| C[Send error response]
C --> End([END])
B -->|No| E[Process request]
E --> F[Send response]
F --> End
end
```

### `Is there already an active request with the same protocol?`

This check is not performed in `Before`. This is taken from the PR in the consensus-spec, which proposes updates regarding rate limiting and response timeout.
https://github.com/ethereum/consensus-specs/pull/3767/files
> The requester MUST NOT make more than two concurrent requests with the same ID.

The PR mentions the requester side. In this PR, I introduced the `ActiveRequestsLimiter` for the `responder` side to restrict more than two requests from running simultaneously on the same protocol per peer. If the limiter disallows a request, the responder sends a rate-limited error and penalizes the requester.



### `Rate limit reached?` and `Wait until tokens are regenerated`

UPDATE: I moved the limiter logic to the behaviour side. https://github.com/sigp/lighthouse/pull/5923#issuecomment-2379535927

~~The rate limiter is shared between the behaviour and the handler.  (`Arc<Mutex<RateLimiter>>>`) The handler checks the rate limit and queues the response if the limit is reached. The behaviour handles pruning.~~

~~I considered not sharing the rate limiter between the behaviour and the handler, and performing all of these either within the behaviour or handler. However, I decided against this for the following reasons:~~

- ~~Regarding performing everything within the behaviour: The behaviour is unable to recognize the response protocol when `RPC::send_response()` is called, especially when the response is `RPCCodedResponse::Error`. Therefore, the behaviour can't rate limit responses based on the response protocol.~~
- ~~Regarding performing everything within the handler: When multiple connections are established with a peer, there could be multiple handlers interacting with that peer. Thus, we cannot enforce rate limiting per peer solely within the handler. (Any ideas? 🤔 )~~

beacon_node/lighthouse_network/src/metrics.rs

@@ -206,6 +206,20 @@ pub static REPORT_PEER_MSGS: LazyLock<Result<IntCounterVec>> = LazyLock::new(||
     )
 });
 
+pub static OUTBOUND_REQUEST_IDLING: LazyLock<Result<Histogram>> = LazyLock::new(|| {
+    try_create_histogram(
+        "outbound_request_idling_seconds",
+        "The time our own request remained idle in the self-limiter",
+    )
+});
+
+pub static RESPONSE_IDLING: LazyLock<Result<Histogram>> = LazyLock::new(|| {
+    try_create_histogram(
+        "response_idling_seconds",
+        "The time our response remained idle in the response limiter",
+    )
+});
+
 pub fn scrape_discovery_metrics() {
     let metrics =
         discv5::metrics::Metrics::from(discv5::Discv5::<discv5::DefaultProtocolId>::raw_metrics());