tracer: handle rc deletion in dynamic config

[ahmed-mez]

What does this PR do?

AIT-9257

Handle RC deletion in dynamic configuration by reapplying the original configuration values.

The remote-config client signals a config deletion by sending a nil content as documented here

// Set the payload data to nil for missing config files. The callbacks then can handle the nil config case to detect
// that this config will not be updated anymore.

Motivation

Bug fix.

In addition to the unit test, this fix has been validated e2e manually and a parametric test case will be added in the future.

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.
• Add an appropriate team label so this PR gets put in the right place for the release notes.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-12-27 09:19:50

Comparing candidate commit 67e0c5a in PR branch ahmed/handle-rc-deletion with baseline commit f986cc5 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 39 metrics, 2 unstable metrics.

[ahmed-mez]

/merge

[dd-devflow]

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[dd-devflow]

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