feat(instrumentation/rum): RUM injection POC

[darccio]

What does this PR do?

Implements an on-the-flight injector to include RUM's snippet in HTTP responses.

Benchmark

goos: darwin
goarch: arm64
pkg: github.com/DataDog/dd-trace-go/v2/instrumentation/rum
cpu: Apple M1 Max
=== RUN   BenchmarkInjectorWrite
BenchmarkInjectorWrite
BenchmarkInjectorWrite-10        1066776              1104 ns/op              16 B/op            1 allocs/op
PASS
ok      github.com/DataDog/dd-trace-go/v2/instrumentation/rum   3.603s

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-09-03 15:01:33

Comparing candidate commit e78b666 in PR branch dario.castane/x/inject-rum with baseline commit 1921d92 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.

[datadog-official]

⚠️ Tests

⚠️ Warnings

🧪 2 Tests failed

❄️ Known flaky: TestSpansStartedTags from github.com/DataDog/dd-trace-go/v2/ddtrace/tracer (Datadog)

Failed

=== RUN   TestSpansStartedTags
--- FAIL: TestSpansStartedTags (0.14s)

❄️ Known flaky: TestSpansStartedTags/default from github.com/DataDog/dd-trace-go/v2/ddtrace/tracer (Datadog)

Failed

=== RUN   TestSpansStartedTags/default
    metrics_test.go:111: 
        	Error Trace:	/home/runner/work/dd-trace-go/dd-trace-go/ddtrace/tracer/metrics_test.go:111
        	Error:      	Not equal: 
        	            	expected: 0
        	            	actual  : 1
        	Test:       	TestSpansStartedTags/default
    --- FAIL: TestSpansStartedTags/default (0.08s)

ℹ️ Info

❄️ No new flaky tests detected

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