feat: Increase trace payload limit from 2 MiB to 50 MiB (#902)

# This PR
Change the trace request limit from 2 MiB to 50 MiB.

# Motivation
When the Node.js tracer layer sends a request to Lambda extension that's
between 2 MiB and 50 MiB, the extension closes the HTTP connection, the
tracer gets an `EPIPE` error and breaks. (Maybe the tracer should handle
the error better, but that's out of scope of this PR.)

According to @rochdev:
> the agent is supposed to have a limit of 50mb

So let's change the limit on agent side to match the expectation.

# Testing
Tested with Node.js 22 Lambda with this handler:
```
import tracer from 'dd-trace';
import crypto from 'crypto';
tracer.init();

function randomGarbage(len) {
  // low-compressibility payload (random bytes -> base64)
  return crypto.randomBytes(len).toString('base64');
}

export const handler = async (event) => {
  const SPANS = 3000;
  const TAG_BYTES_PER_SPAN = 20_000; // ~20 KB per span tag (base64 expands a bit)

  const root = tracer.startSpan('repro.root');
  root.setTag('dd.repro', 'true');

  for (let i = 0; i < SPANS; i++) {
    console.log(`Sending the ${i}-th span`);
    const span = tracer.startSpan('repro.child', { childOf: root });
    span.setTag('blob', randomGarbage(TAG_BYTES_PER_SPAN));
    span.finish();
  }
  root.finish();

  const response = {
    statusCode: 200,
    body: JSON.stringify('Hello from Lambda!'),
  };
  return response;
};
```

### Before:
There are errors like:
```
Error: write EPIPE
at WriteWrap.onWriteComplete [as oncomplete] (node:internal/stream_base_commons:95:16)
at WriteWrap.callbackTrampoline (node:internal/async_hooks:130:17)
```
```
LAMBDA_RUNTIME Failed to post handler success response. Http response code: 403. {"errorMessage":"State transition from Ready to InvocationErrorResponse failed for runtime. Error: State transition is not allowed","errorType":"InvalidStateTransition"}
```

### After
When Lambda's memory is 1024 MB, the error no longer happens.
When Lambda's memory is 512 MB, the invocation can fail due to OOM. But
I think that's a legit error. We can ask customers to increase memory
limit for high-volume workload like this.

# Notes
cc @astuyve who set a `MAX_CONTENT_LENGTH` of 10 MiB in
#294. This PR
increases it to 50 MiB as well.

Thanks @dougqh @duncanista @lucaspimentel @rochdev for discussion.

#899
Jira: https://datadoghq.atlassian.net/browse/SVLS-7777

Author: lym953

bottlecap/Cargo.lock

@@ -49,6 +49,7 @@ opentelemetry-semantic-conventions = { version = "0.30", features = ["semconv_ex
 rustls-native-certs = { version = "0.8.1", optional = true }
 axum = { version = "0.8.4", default-features = false, features = ["default"] }
 ustr = { version = "1.0.0", default-features = false }
+tower-http = { version = "0.6.6", default-features = false, features = ["limit"] }
 # If you are adding or updating a datadog-owned code dependency, please ensure
 # that it has a clippy.toml rule for disallowing the reqwest::Client::builder
 # method in favor of our

bottlecap/Cargo.toml

@@ -3,7 +3,7 @@
 
 use axum::{
     Router,
-    extract::{Request, State},
+    extract::{DefaultBodyLimit, Request, State},
     http::StatusCode,
     response::{IntoResponse, Response},
     routing::{any, post},
@@ -18,6 +18,7 @@ use tokio::sync::{
     mpsc::{self, Receiver, Sender},
 };
 use tokio_util::sync::CancellationToken;
+use tower_http::limit::RequestBodyLimitLayer;
 use tracing::{debug, error};
 
 use crate::traces::trace_processor::SendingTraceProcessor;
@@ -72,7 +73,9 @@ const INSTRUMENTATION_INTAKE_PATH: &str = "/api/v2/apmtelemetry";
 
 const TRACER_PAYLOAD_CHANNEL_BUFFER_SIZE: usize = 10;
 const STATS_PAYLOAD_CHANNEL_BUFFER_SIZE: usize = 10;
-pub const MAX_CONTENT_LENGTH: usize = 10 * 1024 * 1024;
+pub const TRACE_REQUEST_BODY_LIMIT: usize = 50 * 1024 * 1024;
+pub const DEFAULT_REQUEST_BODY_LIMIT: usize = 2 * 1024 * 1024;
+pub const MAX_CONTENT_LENGTH: usize = 50 * 1024 * 1024;
 const LAMBDA_LOAD_SPAN: &str = "aws.lambda.load";
 
 #[derive(Clone)]
@@ -231,10 +234,12 @@ impl TraceAgent {
                 V5_TRACE_ENDPOINT_PATH,
                 post(Self::v05_traces).put(Self::v05_traces),
             )
+            .layer(RequestBodyLimitLayer::new(TRACE_REQUEST_BODY_LIMIT))
             .with_state(trace_state);
 
         let stats_router = Router::new()
             .route(STATS_ENDPOINT_PATH, post(Self::stats).put(Self::stats))
+            .layer(RequestBodyLimitLayer::new(DEFAULT_REQUEST_BODY_LIMIT))
             .with_state(stats_state);
 
         let proxy_router = Router::new()
@@ -254,6 +259,7 @@ impl TraceAgent {
                 INSTRUMENTATION_ENDPOINT_PATH,
                 post(Self::instrumentation_proxy),
             )
+            .layer(RequestBodyLimitLayer::new(DEFAULT_REQUEST_BODY_LIMIT))
             .with_state(proxy_state);
 
         let info_router = Router::new().route(INFO_ENDPOINT_PATH, any(Self::info));
@@ -264,6 +270,8 @@ impl TraceAgent {
             .merge(proxy_router)
             .merge(info_router)
             .fallback(handler_not_found)
+            // Disable the default body limit so we can use our own limit
+            .layer(DefaultBodyLimit::disable())
     }
 
     async fn graceful_shutdown(shutdown_token: CancellationToken) {