Fix Netty 4.1 server span completion for streaming responses

[ygree]

What Does This Do

Updates Netty 4.1 server instrumentation to keep request spans open until the HTTP response is actually complete.

It tracks pending HTTP/1.1 request contexts in FIFO order, finishes streaming/chunked responses on LastHttpContent, finishes header-complete responses such as 204/205/304, HEAD, CONNECT, Content-Length: 0, and WebSocket upgrades at the response headers, and marks incomplete responses as errors when the channel closes.

Motivation

Netty 4.1 server spans could finish when the first response headers/chunk were written instead of when the response completed. That made streaming response durations too short and could misattribute spans on keep-alive or pipelined HTTP/1.1 connections.

Additional Notes

The fix avoids relying on a single channel-level streaming context because that is not sufficient for pipelined requests. Response writes are matched to the queued request context instead.

Contributor Checklist

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  Use solves instead, and assign the PR milestone to the issue
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Jira ticket: APMS-20050

[datadog-datadog-prod-us1-2]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 69.73% (+0.00%)

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[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.00 s	13.93 s	[-0.3%; +1.3%] (no difference)
startup:insecure-bank:tracing:Agent	12.96 s	13.05 s	[-1.4%; +0.1%] (no difference)
startup:petclinic:appsec:Agent	16.84 s	16.65 s	[+0.2%; +2.1%] (maybe worse)
startup:petclinic:iast:Agent	16.91 s	16.92 s	[-1.2%; +1.0%] (no difference)
startup:petclinic:profiling:Agent	16.46 s	16.30 s	[-3.7%; +5.6%] (no difference)
startup:petclinic:sca:Agent	16.81 s	16.63 s	[+0.0%; +2.2%] (maybe worse)
startup:petclinic:tracing:Agent	15.73 s	15.72 s	[-5.8%; +5.9%] (unstable)

Commit: f65d4a45 · CI Pipeline · Benchmarking Platform UI

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